Hi,
i’m having the same exact problem, but i think i solved it in a different way, by adding media_address=external ip address (the same as in the sip configuration) to pjsip.extension.conf, as indicated in the astersik wiki (ht*tps://wiki.asterisk.org/wiki/display/AST/Configuring+res_pjsip+to+work+through+NAT).
Theres no chance to add this parameter via GUI (if there’s i haven’t found), so it has to be edited from console, with the parameter being blanked at every config change.
I restarted asterisk after the edit.
Is your solution working with the extension connected on the local network ?