without the google spyware, you should only use the fdroid version. Not sure what to suggest if you’ve tried it and it isn’t working. Have you tried a different phone? Have you looked at server and phone logs to see if there are any errors?
I don’t know what kind of changes “e.foundation” makes to AOSP, so its possible they’re doing something to break it.
I have the same issue. My server is on a public https and the apache server is a proxy to an internal network server. Everything else, including Nextcloud apps on the phones work and sync without a problem.
However, Nextcloud talk refuses to work from either Android or iOS. I can log in and the app gets stuck at “Grant Access” and refuses to go further.