TelloFpv can't morph itself into the demo version. Tellofpv demo is a completely different app.
TelloFpv normally tests twice for a license: On first startup, and on the next startup >3h later. After that it stores a local license and will never bother you again.
If the app continues to verify the license you have modified the android time settings (see trouble shooting in the manual how to fix)
If you still get the "no internet" problem then this is still the play store problem. I can't fix Play Store for you, sorry, you have to contact Google to get your problems solved.
If you have a modified play store or a custom ROM you are on your own because Google doesn't care.