Hi FredZ thanx for advise. I see you are quite active/helpful here, which is very much appreciated.
I actually found the LinkSys SetUp CD and started the SetUp wizard. It searched for the AP and found it at 192.168.1.244 (which is the IP the IT guys from my father-in-law's company wrote down at the time they set it up). I canceled the SetUp at this point and everything is still running as before.
However: When trying to access this IP by entering it into the browser I do not get any hint of the web interface. Please note that AngryIP / IPscan (192.168.0.0 -> 192.168.1.255) did not find anything at this IP either.
Have you seen / heard this before? How can I access the web interface (ultimate goal is to add a MAC adress?)
Again: I do not want to reset anthing ... It's my father in laws environment and this can go only wrong ;-)