Try one of these:
a) Fully quit Dot3D (Android, Windows: press X Button. iOS: https://support.apple.com/en-us/109359). Then restart Dot3D.

b) Have Dot3D running continuously for > 10 minutes and make sure your device does not sleep during that time.

The reason your license may show as expired is that your device's internet access got interrupted. The key is to get Dot3D to refresh your license.

Normally Dot3D checks the license status at every start and every 10 minutes of operation. Of course it can only do that if it has internet access to contact our license servers.