If you don't want to use the unofficial workaround to add the MS Store back to the LTSC build (since some have reported that not everything in the Store works properly when it is added to an LTSC build using that method), you can just use the MS Store for Business instead. Just download the offline bundle for the App Installer app, and install it, then you can use it like normal to install .appinstaller packages. The MS Store for Business also lets you download any dependencies you need for a given app. See this article for more info on downloading offline app packages from the MS Store for Business.
The MS Store for Business is free - you may already have access via an O365 account if you or your organization has a subscription (but in that case you may need to request an admin in your org to add the App Installer app to it if you are not a global admin yourself). Otherwise, you can set up a "free O365 org" with a personal email address and use that to access it (just sign in with a global admin account for your personal/free O365 org when you go to the MS Store for Business link, then you should see all the options to manage it and enable access to offline apps, as is described in the article linked above) UPDATE 04/2023: This is no longer an option, as the "free MS Teams O365 organizations" that you could create before have been deprecated (all such orgs that existed previously are now deactivated/inaccessible and no new ones can be created, so you need an actual paid O365 account now to access the MS Store for Business). Also, the MS Store for Business will supposedly be retired soon as well - see my update below for details on the new "supported" method for using App Installer on LTSC and Windows Server builds.
Alternatively, you could install the .appinstaller file that you have using PowerShell manually with Add-AppxPackage using the -AppInstallerFile parameter. If installing manually via PowerShell, you have to manually obtain any dependencies and install them first (if it is just the VC++ Runtime Framework Packages, those can be downloaded from here; otherwise, you should be able to find them on the MS Store for Business). You'll also have to enable sideloading of packages or Developer Mode on your machine in Settings -> Windows Update -> For Developers.
*UPDATE 04/2023: Although the above info is still valid (for the time being anyway - but the MS Store for Business will supposedly be retired in the near future), the much easier way to do this now (and the more "supported method" going forward, as MS has officially noted that App Installer and the winget CLI tool are the replacements for the MS Store for Business going forward) is to just install the App Installer package from the official GitHub site (no need to use any workarounds to try to add the MS Store back).
You can download the .msixbundle package from the official GitHub site (go to the Releases page and download the Microsoft.DesktopAppInstaller_8wekyb3d8bbwe.msixbundle file from the latest stable release as well as the corresponding license XML file) - this is the official App Installer package, in offline/downloadable form. This can be used to install App Installer (which includes the winget CLI tool as well) on any Win10 LTSC or Windows Server machine using Add-AppxProvisionedPackage (we must use Add-AppxProvisionedPackage instead of Add-AppxPackage to install the App Installer package - see below for details).
However, there are a couple caveats here:
One is that Add-AppxProvisionedPackage unfortunately will not automatically resolve dependencies, so you need to download those yourself and either make them available in your installation command or install them manually first (prior to installing the main package). The dependencies needed are: Microsoft.UI.Xaml.2.7.appx Note this must be v2.7.x (v2.7.3 is the latest stable release of that line at the time of this writing) - using v2.8.x will not work. This can be downloaded from here (the download is a .nupkg file; change extension to .zip, extract, then grab the Microsoft.UI.Xaml.2.7.appx package from the \Tools\AppX\x64\Release folder inside the extract). Microsoft.VCLibs.x64.14.00.Desktop.appx This can be downloaded from here. Once you've obtained the necessary dependencies, they need to be installed with Add-AppxPackage or Add-AppxProvisionedPackage (you may also be able to just install them at the same time as the main App Installer package by utilizing the -DependencyPackagePath parameter with Add-AppxProvisionedPackage, but I haven't tested that). Since the App Installer package itself has to be installed machine-wide (because we have to specify a license file - this is the XML file you downloaded from the GitHub site along with the .msixbundle file), it makes sense to install the dependencies machine-wide as well (otherwise you may have problems when the staged App Installer package tries to install for new users that sign in). To install the dependencies for all users, use Add-AppxProvisionedPackage - here is an example: Add-AppxProvisionedPackage -online -packagepath "C:\Packages\Microsoft.VCLibs.x64.14.00.Desktop.appx" -SkipLicense add-appxprovisionedpackage -online -packagepath "C:\Packages\Microsoft.UI.Xaml.2.7.appx" -SkipLicense The good news is, this is the only time you should need to worry about this - once App Installer is installed, you can install all new MS Store apps with winget which will resolve all necessary dependencies automatically for any packages you install. The other is that you apparently do need a license file when installing using this method or you will run into an issue documented here. Because of this, and the fact that Add-AppxPackage doesn't have a -LicensePath parameter, you must use Add-AppxProvisionedPackage with the -Online and -LicensePath parameters to install the main App Installer package (which means you are installing it for all users, not just your own user). Example command for installing the main App Installer package (after installing the dependencies as noted above): Add-AppxProvisionedPackage -Online -PackagePath "C:\Packages\Microsoft.DesktopAppInstaller_8wekyb3d8bbwe.msixbundle" -LicensePath "C:\Packages\3463fe9ad25e44f28630526aa9ad5648_License1.xml" -VerboseOnce installed, you can then either download .appinstaller files and double-click them to install like you normally would, or you can use the winget CLI tool to search for/install/upgrade/manage MS Store apps from the command line (this gives you access to any and all apps that are in the regular MS Store. I've found the CLI method to be the easiest way to manage MS Store apps on LTSC builds - just use winget search [package] to find the package you want, then install or update with winget install [package] or winget upgrade [package]. Installed packages can be listed with winget list. I've used this process for a while now on various LTSC machines and it has worked great.