You're adding more variables to the equation and there were already too many unknowns. :)
If your phone connects as expected on external wifi, but not on the LTE network then it would seem to point to an issue with your mobile carrier, but you'd have to do a lot of troubleshooting to confirm that assumption. I'd start with ping tests while connected to LTE and if you can't ping, do a traceroute from the phone to see what's going on. Is it possible your mobile provider is blocking OpenVPN connections? Unlikely, but maybe – again too many variables & unknowns, not enough facts.
But wifi works and LTE doesn't is at least something to work with. Do the ping & traceroute tests when connected to LTE and work from there. Seems like a phone config problem. Does your LTE provider do CGNAT? I don't think that should cause an issue, but maybe? Have you tried connecting with another device?
Having connected successfully to both the webgui and your rdp host behind pfsense using wifi, I think it's safe to say the openvpn server is properly configured and operational. The rest, I'm afraid, is going to be up to you to troubleshoot.