Zero Tier and RDP on Android


I have a private zero tier network working 11 devices.
It is all working well and I can RDP from all other windows PCs on any network.

I have installed zero tier on my android phone.
I can ping the PC from my phone and the phone from my PC on external network. So the connection is working.

I am trying to use the windows Remote Desktop 8 app on Android to logon to a PC from outside my network.

I can RDP from my phone (android) when connected on the same router, so RDP is working.

But when I try use mobile internet the RDP connection wont work, as I assume the app is not using zero tier to look for the connection and is using the mobile internet.

There is an option to add a gateway address in the RDP app. I tried some addresses and no luck

Any advice on how to get it to work would be much appreciated?