Seems I figured it out.
The problem is the default settings o fthe RN2483, in the docs there are default channel parameters given which I expected to be set if the device is factory resetted but this is not the case.
Actualls 8 channels are active and used by the device. But the gateway I set up using Gateway OS respects the settings of the gateway profile which has only channels 0-2 active.
Using the RAK gateway all 8 channels are listening and it does not use the settings by the app server so… And so does the TTN gateway I also tested…
Think this is solved!