UAVcast currently supports Zerotier and OpenVPN without the need for any additional software running on the Raspberry Pi.
VPN page Overview
Zerotier establishes a direct connection between the connected devices, even if they are behind a NAT.
Depending on your cellular providers NAT policy - if you are having issues with video streaming over UDP, try using a TCP for your connections or try OpenVPN.
OpenVPN connects devices togheter using server in-the-middle principle. Many cloud services has prebuildt servers with openvpn for easy startup.
When you have started your vpn access server, login and add a user / client. For the user you want to asign to UAVcast, make sure you have ticked the auto-login box.
Now, login with your newly created user https://your_openvpn_ip:943/
Download the (Yourself- Autologin profile)
Drag this file into UAVcast-Pro => vpn page and hit connect.
For more information about setting up OpenVPN, please see the Open VPN Wiki.