Multiple servers inside private network


I have identified a potential issue with a setup I am experimenting with, and was wondering if someone could help:

imagine I have two servers inside a Private Subnet:

Asterisk primary: (GW
asterisk Secondary: (GW

both have static IP addresses, and both need to register standard SIP trunks to an ITSP.

Secondary needs to register to the ITSP services, however the GW Port forwarding is set to sending port 5060 to Primary, so secondary cannot be registered using that port - is there a way around this, considering the ITSP will only support 5060?

IE how can I register multiple trunks from Multiple PBX’s inside a private subnet to an ITSP?

Strange ITSP. FInd an ITSP who doesn’t impose arbitrary restrictions on the contact address for registrations.