I tried some more stuff and realized that the error message shows up when I try to create more than one rule for the same port.

So the error message is wrong.

But that brings me to the same question again why am I limited to 5 source IP addresses per port?

@tmtint Do you find the solution somehow? I want to addd 8 ports tto forwarding and i have the same issue. 

@tmtint i might have solution,

after i turn on LLDP on the Gateway i might to add more than one port forwarding. 

Thanks for letting me know. I never heard of LLDP before but read into it.

Do I understand that right that it shouldn't have anything to do with it? Great that you found out how to make it work though!

I already switched to another router because the software is so immature.

Gonna keep an eye on it though, because I still use an Omada AP and also have kept the router to see if the software gets better.