I am been working on this for weeks and also ask on the Zerotier forum but no one can help me.
I have two office connected to the internet with a 4G modem in bridge to the OPNSense router/firewall.
I installed the ZeroTier plugin and configure everything like suppose to be. Below my setup:
Never used Zerotier but my understanding it that it sits on top of your existing connection so if Zerotier is working then it’s not going to be a router / firewall problem. It’s going to be a zerotier problem.
as I say, I don’t use it so I don’t really know but I think it works the same as other vpn ish solutions.
Again, I could be way off here but my understanding of zerotier is that it’s for connecting multiple devices together not for connecting multiple sites.
In my opinion you would be better off with a pfSense box (or VM) running OpenVPN (or wireguard now I guess)
OpenVPN is pretty stable, site-to-site just works, though I much prefer running two RAS. If you are running OpenVPN 2.5 I believe they have updated their encryption and security further, that’s more important for sure when connecting businesses.