I'd suggest to have a look at the manual, trouble shooting section:
TelloFpv keeps asking for internet access to validate the license even after a few hours
TelloFpv validates the license for the first few hours after installation. After about 4hrs it stores a permanent encrypted license on the device and uses that license instead. Exception: If you changed the Android date / time settings and disabled “Network provided time” then a local license is not stored. Such a user defined date makes it impossible to ensure the Play Store cancellation period has passed. To fix: Re-Activate “network provided time & date” in the android settings. Start TelloFpv. Wait at least 4hrs. Start TelloFpv again and a permanent license is stored. Now you may change the date & time settings to your liking.