One port out of a set range is blocked?

Using a Zyxel ATP100

We have 5 servers set up to run cameras for a client. In order for the feeds to reach the worker’s computers or phones, we open a range of 10 ports which covers everything we need. For example, on server 1, we open 11000-11010, server 2 we open 12000-12010 and so on.

For whatever reason, 14008 is not accessible. I’ve had a few talks with Zyxel and they confirm it is set up properly, but this one port is giving me an issue. I test it with canyouseeme.org and the mobile camera application itself.

Things I have tried, but not worked: 1. Delete and re add the rules for the port forwarding 2. Create a rule for that specific port 3. Changing which port is used, and making a rule to match it

What else could be blocking it?