If it's your own server and you enabled whitelist and you're OP, you should be able to join.
If not, you can go to the server console on the dashboard and type whitelist add (your username) or whitelist off or op (your username).
If you want to upload a custom nether and end (I think that's what you're asking...), you need to first delete the current world_nether and world_the_end, then upload the new ones with these names.
The only 1.16.x version is a dev build and apparently contains bugs, best bet is to wait until it's been publicly released and then suggest it as I doubt Minehut upload dev builds.