Is it a rule that the time spent on adverts don't count, or is turning off internet for the device allowed?

Turning off WiFi is allowed to prevent adverts, yes.

Usually ads keep appearing even with WiFi turned off. What should I do about that?

@Javi17 If you are playing on Bluestacks, turn WiFi off and then restart Bluestacks. If you are playing on an actual Mobile, make sure you have Mobile Data off, and if it keeps happening, restart GD.

Ok thanks

You can also just use Airplane Mode