Hi, I got this working. This is what I've done.
1. Edit container settings and change configuration path to look like this (you point to the directory, not to the file, that was my problem)
2. With windows or whatever edit a file named turnserver.conf with this content (It is a simple config, you only have to change the secret and the domain name)
3. With krusader copy the turnserver.conf you just created to /mnt/user/appdata/coturn/
4. Forward the ports in the router as OP says
5. Restart the container
6. In nextcloud add the servername:port and secret and TCP only
Now click to test and must be successful.
BTW @xthursdayx thanks for you work