We don't use the license.xml file and I'm not even sure how it's even used, it's likely some feature that is used by other products that use QLM/Soraco, so you must have bumped in it for some reason, but it's not something you are supposed to have normally.
The one and only way to activate our products is with the "Activate" button of the FSDT Installer OR the Activate function in the Addon Manager.
The likely reason why your activation doesn't stay doesn't have anything to do with the license.xml, it's usually caused by running FSX in a wrong Compatibility mode (like XP/Vista/Windows 7) under Windows 10/11 or, possibly, using the unsupported Windows 7, for which we have Sticky a post explaining how it *might* work, but we don't offer any support for it.
If you use a supported OS, FSX should not run in any Compatibility mode, which is how is installed.
Another possible reason why the activation doesn't stay, is if your Windows user account has limited permissions. The easiest solution would be in this case to give admin permission to your user.