My understanding is that Syncthing can handle multiple layers of NAT. Does this sound right?
After poking around the Zerotier documentation, it insists that any firewalled LAN you want to connect to shouldn’t be NAT’ted more than once. Zerotier seems to only be designed to handle one level of NAT gracefully (without the need to relay all traffic). Does this also sound right here?
I would never design a LAN with more than one layer of NAT myself, but I strongly suspect that some ISPs will impose one or more additional layers of NAT on you, that you can’t avoid (especially 4G ISPs).
Much as this is an interesting thing I’d like to keep the development category here on the forum somewhat on topic. Please keep the discussion relevant to Syncthing, at least tangentially.
I suspect they live under the same limitations we do - double NAT traversal may work under some circumstances if you’re lucky, but there’s no systematic way to make it reliable. Certainly there’s not much we can do about from the end users perspective behind all the NATs.