Gaming / Suddenly Bad Internet? Delayed + jerky?

Watching Netflix last night started slowly. The picture was no longer smooth, it jerked. Right now I want to play Csgo, which usually works fine. But this also jerky, even so much that it is no longer playable. What can I check or what can have happened suddenly and how can I fix it?

Restart the router, maybe this solves the problem.

If necessary, you can simply use a LAN cable if the router is still jerky or the network provider has a problem.