hello bbay
You’ll have to check if you get a true public IP on the Connect Box 3.
Log on to the Connect Box 3 and have a look at Admin > Info
Then go to https://whatsmyip.com

If IPv4 address and the result shown are the same, things should be ok. You might have to restart or reset the Connect Box 3. If they are different, you’re on a CGNAT connection and port-forwarding will never work for you.
In that case you can call 0800 707 707 and request a non CGNAT connection.
In case you have a IPv6 connection you’ll have to request a IPv4 connection.