When your ISP doesn’t really believe what you tell them, and tells you the issue is not on their end.
I am not naming names, even the ISPs involved – I’m fairly sure this is just a misplaced ETH-cable. But I will let you know once I get a solution one day – I’ll post an update in the comments then.
For now, I’m immensely happy with myself for deciding to keep a mobile connection from another ISP as a backup. This decision saved my weekend, and allowed me to get my weekly dose of p… cartoons.