UAVcast currently supports Zerotier and OpenVPN without the need for any additional software running on the Raspberry Pi.
Zerotier establishes a direct connection between your devices even if they are behind a NAT. Zerotier is extremely easy to setup and is free to use.
For more information about Zerotier, please visit Zerotier.com
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 routes all traffic between your devices through a server.
You can setup a OpenVPN server or find a provider that can host it for you.
For more information about setting up OpenVPN, please see the Open VPN Wiki.