@Popester69er Hi, unfortunately it looks like our standard steps have already been taken.
From the description it sounds like a router issue, either related to config, or an issue with the hardware itself.
You may have done so already when talking with your ISP, but power cycling your router (turn off and unplug for a minute, then reconnect) may help.
Just to be 100% sure, you can try creating a new account and logging in with that to test, but I suspect the issue will still manifest.