Double-nat punching through service running on router

I have an OpenWRT box that I’m planning on using to multiplex one connection for all the devices running on my RV. Having a local network like that is great because it means that all the services running on my RV can communicate through my router, instead of some RV park router that generally isn’t reliable. It also means I that I don’t have to change the network settings on every device.

Unfortunately zerotier won’t work with a double NAT. It would be great if there was some service I could install on the openwrt router to let zerotier know if can send requests to that router (using normal hole punching) and the router would be able to forward them on to the clients. Some kind of assisted NAT punching so that zerotier works behind that double nat.

I’d be very open to any other ways to solve that problem, but so far I haven’t found any.