I would first check to see if there is new firmware for the router and if not try reinstalling the current version of the firmware. The procedure for reinstall of the firmware is a bit different in that you load the firmware onto your computer, disconnect from the internet, and do the install. You should be able to find instructions for your specific router. You may need to look for something like "installing previous version of router firmware"

I've installed the latest (as of two weeks ago). You say I should reinstall it?

@kevinp1335 

 

Another thing to try is to use a different web browser as something may be cached.

I normally use Firefox. I tried Edge. Still got the "Login Failure" message.

Some have corrected weird issues by reinstalling the firmware. If the current version worked fine then try reloading it.