If you know where to look on internet you can get millions of WiFi router factory default user and password information.
The ones on the label.
Run a program that will test all the showing routers in your area and maybe if you get lucky your on.
I always change this ever time we get a new one as easy to do. Much more secure...
You need a wired connection. So ethernet cable from pc to router.
Type in pc browser (chrome or edge or whatever you use) the address which will be on the router label.
Look's like.. But not this..192.001.09.01....
That brings up login page.
Follow with user and password also on router label.
Change to your own user and password.
That's it.
Just need to sort out all you other devices with the new WiFi password as you just changed it.
As for your problem I don't know. Maybe a device has logged on your network.
Just reset it all by changing user and password.