Hallo,
In my case, I have on the remote extension (i.e. Express Talk in Network design) set SIP on 5080 and RTP 8000.
In the router on Axon site I have a portforwarding/porttranslation UDP 5080 (81,82,82) to inside LAN IP address 5060 (61,62,63) [[i use port 5060 WAN site on router for a other Voice application]]
In de router on Axon site I have a second portforwarding UDP 8000,01,02,03 to insite LAN IP address 8000,01,02,03
In the Axon PBX I have in Network design SIP 5060 and RTP 8000.
So I hope it work, perhabs you can set in remote extension and in router SIP port UDP 5060 in case 5080, please try it.
Succes, regards Cor