Routing between two UniFi USG based LANs with ZeroTier
This is a quick explanation of how to configure both ZeroTier and two Ubiquiti UniFi USGs to allow routing between two IPv4 networks. Both networks are in private (RFC1918) address space and each one has its own DHCP service. There is no need for NAT between them, only IP routing.
The two networks are
192.168.1.0/24 (call this the "left" network) and
192.168.10.0/24 (the "right" network) but they can be anything. Also, you can have multiple CIDR blocks on one side or the other of the ZeroTier route; if you do, you just have to create more routing table entries.