Just to update everyone on this, in case someone has a similar issue.

The ISPs would not block the ports for us, which I suspected. So with the help of some members on this forum we setup firewall devices yesterday for both of our IPU cards.

Specifically the ZyXEL ZyWall USG 20 using a setup generously provided by newtecky.

It hasn't been long enough to determine if this has solved the issue, but I will update everyone in a few days with the final results.

One nice benefit of using this device is that we are able to see logs for the first time, and the attempts made to access or query the device.