Since April I have noticed that something is wrong with my internet connection because I bought a switch and wanted to connect it to the internet. This was not possible because no connection could be established although I have given the correct password. I did not think anything, but then new devices could not log in, but my phone or laptop have had normal Internet. I bought a new phone a few days ago. Wi-Fi is displayed, but when I create an apple id I'm always shown that it can't connect to the server. Since yesterday is that now synonymous with other devices that have previously always worked. I'm at Telekom and my router is 5 years old.
Did you try your router for a few seconds (40 seconds) to disconnect from the power and then reconnect?
Sometimes such a reboot will do a lot.
You could also open the interface of your router in the web browser and see if there are any errors displayed?