The subject was changed, I guess it’s the fastest way.
Actually since the second PBX which accepts registrations is reachable via a public address I would try to keep it safe as much as possible. The first PBX is behind a NAT and it’s not directly exposed.
The problem with this scenario is that the first PBX doesn’t have a static IP and I can’t use DDNS for it. It may not even be possible to link the PBX’s like this but I wanted to know for sure before I try something else as it’s the most direct way.
Actually the thing that led me to try this option was a post by @lgaetz in this thread
Configure the PBX with the dynamic IP to register to the other, and config the static IP to accept the registration. Exactly the same as if it’s a phone.