it is easy to do with wireguard but you need a server with an internet routeable address
very handy for me doing relay testing with the help of a reverse proxy forwarding inbound relay traffic to my dev machine
Discussion
tailscale, openvpn, etc. There are many ways...
it's literally impossible to do it without one side having inbound routeable connection, that's just how it is
without universal IPv6 that's gonna continue to be a very common situation
the mobile operators have had plenty of time to get that sorted out, so i call conspiracy and cartel on it