Get working config only if public IP is entered instead of Dyn URL

I get a warning “i” in Talk App Settings when entering my.turnserver.domain:3478.

Giving my.public.IPv4:3478 then i get a tag that it’s working.

My Router has opened port 3478 UDP and TCP and forwarded to the turnserver.

My nextcloud server is working behind the same router and behind a reverse proxy because i have different services using port 443 (cloud, mail, …)

My turnserver URL (turn.my.domain) isn’t routet through my reverse proxy. At DynDNS my turn.my.domain points to my public IP adress.

Can anybody help me with that, what’s wrong?