The problem of the activation not sticking, is usually caused by having mistakenly set a Compatibility mode for the simulator, like XP mode under Windows 8/10, which is wrong and not how the simulator is supposed to work normally.
Go into the Properties->Compatibility tab for Prepar3d.exe, and be sure nothing is enabled there. The only setting that we don't require, but can be used, is run As Admin.
Also, be sure you don't start the simulator from another product, because this can cause the same problem. Start the simulator from its own icon, at least for the activation.