As Sera67 said,
Avatars can be summoned in Single player or some private servers if the admin has it turned on. I believe in officials this is currently turned off.
Mummy of the Ring does not exist currently in game. He can be spawned in by admin command only, check the wiki for the code, although I know you’re trying to do this without admin commands. Currently it is not possible to get this step without admin spawn.
Explore the exiled lands was apparently fixed in a recent patch but I have not yet tested it. About three months ago I attempted to get it on PS4 but was unable to and reported the bug. Since then it has supposedly been fixed, so let me know if your able to unlock it. Do note that they added a location in the jungle (“slithering Island?”) maybe it’s the one with all the snakes as part of the fix. So don’t forget that one.
Hope this helps, and let me know how it goes.