I can not connect after being connected successfully in the past:
After a certificate renewal (90 days) you may see your app fail to establish a connection to the server or you may see its connection state cycle on and off. Android users may get various failed to connect warnings. Deleting the server and setting it up again will generally resolve this issue by forcing the app to get the new certificate.
IPHONE Users have a new feature in settings - Network - Server - Select ATXTAK server - To refresh the certificate which will resolve the certificate expiration and update with a new certificate.
I am connected but can not see anybody else on the server:
Make sure you have the channels toggled on and they match the channel being used with the other users.