Hopeless ISP and port forwarding will not work

Hi guys

Well, it seems like I have come across the most helpless ISP in the world.
When asking them a simple technical question I got the reply: "I'm sorry but we are not able to help you with these kind of questions. I would suggest that you reach out to a company dealing with similar things".
I then followed up by asking if there were any other departments that would be able to help, but "No, I'm sorry, this is the only technical department".
Needless to say I was quite shocked.

My question was rather simple, can you help me with port forwarding? Not even getting into the details.


Model: Zyxel AM3100-B0
Firmware: V5.17(ABNY.7)E0

I have created a port forwarding rule, saying that port 21 should forward to my FTP-server. The FTP-server is working (tested by FileZilla to local address, as well as everything working before changing ISP)
I have tried all available WAN interfaces inside the port forwarding rule (Default, MGMT and INET)
I assume INET is the correct interface as this WAN has my public IP.

Does anyone have ideas for me to test?
I'm starting to think that my firewall could be causing problems? The problem with that is that this router is not very configurable - I only get to see that the firewall status is "Medium", and have no way of changing it, or any settings within it.

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