Why doesn't Zyxell Zywall USG 300 allow port forwarding with an arbitrary number?

Question

ZyXEL

Good afternoon, there is a model of the Zyxell Zywall USG 300 hardware gateway, the task is to forward the port to the outside, in principle it is done, but it is impossible to assign an external port to some non-standard one. In my case, I forward 5900, and only then I specify 5900 as an external port - everything works, when it's different, then no!
PS I also changed the rules on the firework - the result is the same
Help with this issue