I am not a tech person but have been using zerotier for a while now on Win10.
Its working but slow.
I am using zerotier in an app connecting two machines that both are in China and both using China Mobile 5G networks. I think the reason its slow is that although all traffic between the machines are in China the zerotier server is in the US so traffic needs to pass the wall.
Yes I have access to the settings of the 5g modems (can connect to them via wlan from a pc).
One machine is a RPI 4 and the other one a pc on Win10.
The app I am running is UAWmatrix. I could try asking in their forums. Just started here since I think you may have a solution.
Anything in the settings you like me to check?