Look at your server logs and make sure that POST requests get routed to the lemmy backend, not the frontend. The frontend does not proxy requests to the backend if the frontend doesn't handle it.
Check your NGINX logs and look for POST requests to /inbox
as well as /.well-known/webfinger
.
You can unsubscribe and subscribe again while tailing the logs to hopefully see the request come in, and make sure it's a 200. If it's a 400 or 404, triple check it's not going to lemmy-ui.
Been there, done that.