I am with Fibia also. There are alot of combinations with two routers you are going to have to experiment with. I would start with turning UPnP off at both the Fibia router (Icotera?) and your own internal Asus router. Then manually port forward using the values on the roon Arc configuration page
Icotera → Asus → Roon Core
You will probably have to port forward on your firewall as well. This is a step I think many are missing.
Fibia do not use generic router passwords so you will either have to ring them up to get yours or you can find it on your Fibia home page.
If you cannot get that to work them removing your Asus router and ringing up Fibia to take their router out of bridge mode should work (that is how I am configured at the moment). You probably do not want to do that because you will have a good reason you are using an internal router. But it will eliminate variables and give you a simpler starting point for diagnosis. If your phone is on Android 12 you will still probably be out of luck. Arc initially appears to work but after pressing a few navigation buttons it will hang. I have had to keep rebooting the phone so Arc is not really usable at the moment.
There seems to be several threads starting. Here is one:
The Android eco-system is quite varied. I am using an Oppo dual SIM phone and I seem to be on an operating system called ColorOS 12.1 which is some kind of customisation of Android 12.0.
I won’t be starting a thread as I don’t have the time for this but anyone trying to get Arc working in Denmark on devices commonly available on contracts in the local market here needs to be aware that there might be a device dependency as well as server side and network configuration issues.