You can’t stack +4 cards, that’s not how it works

Well, as you said, the reason could be either your router or on the ISP side. If you have another router you can test (maybe from a friend?), that would be a good first start. Otherwise, you’ll probably have to call out an ISP tech to see if they can identify any issues.

Lifehack for you: since it worked out this time, you can wait even longer next time

Dann doch der Freitod an den Bahngleisen oder mit dem Einweggrill. Das ist natürlich deutlich besser