I think Access Control can meet your requirements. If you want to block all the Access, when you set the Target you need to select the IP address and then block all the ports like this picture shows....

The new firmware also can have the admin account. It did not change the admin account but add the guest network to the main one.

Not quite sure about your peoblem or question. The default login username and password are both admin. If you have changed it, no one know what it could be until you reset the router to the factory...

Are you using the guest account or using the account you defined? Maybe the default guest account can do the job.

My first idea is about to turn the hardware off to see the problem can be solved or not.

You are welcom:) We come into the forum and help each other to solve the problems we have met.