For starters, can you confirm, that you have a public IP address?
Is your mobile interface in Network -> Interfaces section assigned with an actual public IP, not a CGNAT'ed IP address, starting, for example, with 10.x.y.z or 100.x.y.z?
Also, I've checked the manual, and what you need to configure is not simply an open port, but port forwarding rule in Network -> Firewall -> Port forwarding section, where you need to enter:
- Select Protocol - TCP;
- External port - 1935;
- Internal IP - IP of your bridge;
- Internal port - 1935.
By default, zones should be set correctly: source zone - WAN, internal zone - LAN.