I was able to do it on my AR750S by doing the following.
- install zerotier package
- edit /etc/config/zerotier add zerotier network id and change enabled to 1
- reboot
- in luci create ZT0 interface, unmanaged and mapped phyical address custom ztqu3bvy4n (check your exact address for Zerotier with an ifconfig from CLI), save and apply
- edit the interface and create new VPN zone Zerotier, save and apply
- go to vpn zone, leave as accept, accept, reject
- I turned on masquerading and MSS Clamping, and added lan and wan to both source and destination zones, save and apply.
After that I was able to ping to and from other devices in my zerotier network.
I then set one device as a bridge and added the route in the zerotier portal and could access my lan.