I don't know what else to suggest.
@licquorice Will getting a 3rd party router help?
Possibly, but there shouldn't be any problems with forwarding ports on the BT Hub. As previously stated, I can forward port 25565 successfully on my hub.
You can try contacting customer services to persuade them your hub is faulty and get a replacement.
0330 1234 150
Did you factory reset the hub by pressing the recessed button at the rear with a pin for about 20 seconds until the lights flash?