Cannot connect to the internet. Also LAN Clients are unreachable

I was at university when I got a text message from my sister, that our internet at home does not work. Once a month or so, the Modem (provided my A1) crashes and needs to restart. I told her to do so, but the problem kept on. When I got home, I called A1 if the internet is down, but they said it was up. When I then could not connect to the A1-Modem via the browser, I knew there was something going one and needed further investigation. The "diagram" below shows the network.
1 answer

Check all cable connections

The solution was hard to find and quite embarissing.

It was so strange, that I only could connect to the access point, but not to any other clients and not to the A1 modem. I checked all cable connections (it was a dark room). I found out that a black thin ethernet cable was connected in port 1 and 2, creating a loop.
I asked my sister about it and she thought that there is a loose cable that should be connected, but was there for maintainance purposes.