Zerotier on GL-MT300N-V2 not showing in firewall 'external zone' list

I have a several Slate AR750S, and helping a friend whom has a several Mango GL-MT300N-V2 set up zerotier. On my Slate with V3.212 firmware, in the firewall settings I can assign the open ports to zerotier. On the Mango with V3.211 firmware zerotier doesn’t show up in the ‘External Zone’ list however zerotier does connect, and he is able to reach the mango from his phone with zerotier on/wifi off. The use case is to reach a device connected to the mango’s lan though,so we need port forwarding. Are we missing a step? or is zerotier just a node on the LAN now? On both devices zerotier is on version 1.6.5