You would normally use the DDNS service on the router at the 3CX end. The "mobile" phone, (remote extension) need to know where to go to register with 3cx if you have a public IP that can change. Setup the mobile set to use STUN and 3CX will know where it is and what port it is using behind a router. If the mobile set is getting a public IP, then you don't need to use STUN.