Generally in professional online tournaments there is not a tournament official onsite. Players are competing from their homes. Instead, tournaments require several Zoom sessions from multiple angles in an attempt to verify that the neither the player's computer nor a secondary device are being used to cheat. These Zoom sessions are monitored by tournament officials.

On top of this, commercial chess websites have extensive anti-cheating measures that are used to analyze the games after the fact. For example, one of the major players has 5+ engineers and several strong chess players on their anti-cheating team. These teams have caught professional players cheating a surprising number of times. Being caught results in a lifetime ban from the chess website and there are often consequences for the player in real life as well.

I don't really buy the idea of an adjournment. Are you supposed to have one every time a connection problem happens? Historically players knew at what point in the game an adjournment would happen. Having them happen at random throughout the game would change the whole dynamic.

Your idea of a cellular connection as backup is an excellent one. I think the first commercial chess website to implement a turn key way for players to utilize one will see huge returns from that investment.