I have many of my remote users (remote desktop to office pc) that just quit working. The solution is, on the remote end, to uninstall ZeroTier and delete all the folders under Program Files, etc. then reinstall to get a new Node ID. After approving the new Node it works. This is getting very tiresome and frustrating for users. I have a SonicWall TZ400 at the office. Is the firewall screwing with it?
There is a nice guide regarding firewall and router configurations here:
https://zerotier.atlassian.net/wiki/spaces/SD/pages/6815768/Router+Configuration+Tips
The key thing, though, is to ensure you are not dropping incoming UDP traffic on port 9993, and the device concerned can send to any net device without traffic being dropped; due to Zerotier ports being somewhat dynamic.