I downloaded the mobile app (ios) and i don’t see any way to connect it to your own selfhosted server. You can only create an account with them. Didn’t look further, but it would be pretty weird to first have to create an account with them and only afterwards being able to connect to your own server.
Edit: The access is just deeply hidden. You have to tap 7 times on the login in screen in the app to enter developer settings. There you can enter your own server.