Please provide a network diagram. You appear to be trying to bypass FreePBX, which doesn’t make sense to me. In a normal system, the only address the provider needs is that of Asterisk itself, which would, near to out of the box, be port 5060 on your public IP address. If your public IP address is not stable, you need a better ISP.
Literally having a static address for the extension is something done in the phone, router, and DHCP server, not in FreePBX.