Hello ziomario,
There is known issue, when you change GW port from GUI, and after that changed service status, like restart/stop/start, it won’t save that port.
However, when you change GW port from GUI and then simply “go back”, without changing service’s status, it should works.
So if you could please try this one – open GUI, change GW port, and just after it simply “go back”. Then return to that section and check if changes was saved?
For other ways how to change GW port please refer to https://www.nomachine.com/AR10R01099#4.1