I was just doing patterns at OIIE when the controller switched. I was on Unicom for a period when no one was active, then someone re-took over when I was on base. They sent me an On Guard message but I was unable to find the frequency in the ATC tab or on the map, so I left the flight, fearing I was going to be ghosted.
I have an iPhone 6 if that makes a difference. IOS 11.4.1.
When your in the top left of your ATC board it say Unconnect from whatever frequency and you press that. Than you will select you new frequency you want. If that doesnt work its probaly because you lost connection somewhere in the flight and didnt fully reconnect to the game even if you can see planes and it shows all green in the top right. That’s what happens to me whenever I cant connect. If approach hands you over and it doesnt work then you probaly have to back out or do what I said above.