I had that same issue when my camera phone was using a router wifi connection that was connected to my modem via ethernet. They both had local network IP ranges that were different. I could not connect to the camera unless I was on the router IP range, nothing else worked.
You don’t by chance have the same setup do you? I connected my camera to my modem wifi and since then I can access it from 3G, 4G, any wifi or a wired connection to the modem.
EDIT: my old setup was MODEM (with 192.168.1.x subnet) -> ethernet cable to wifi router (with 192.168.0.x subnet). The camera was behind a 2nd subnet with double NAT from the modem and router combined. I could not access the cam from my PC while connected to the modem. I plugged into the wifi router and it worked though. I removed the router and using the modem (has wifi built in) only now with no issues. The only way around this setup if your modem does not have wifi on it, is to bridge the modem and let the router handle all the other stuff.
If this doesn’t help, sorry for wasting your time reading this.