Unable to access routerlogin.net

Router is 3 yrs old has worked fine until recently. My wife's laptop can only access the 2.4 connection, mine can connect to both 2.4 and 5.0.   At intervals recently, the 2.4 connection does not show up as even available on my computer's list of available networks, and my wife cannot connect to the internet.  AT other times, both connections work fine. Streaming on our smart tv has also become occasionally unreliable - the stream interrupts for several seconds then catches up.

I have the led lights on the router turned off; I tried to get into the settings by logging into routerlogin.net to see if that would give any clues, and am unable to do so. I get a message I am not connected to my router - even though I am accessing the login page via that router, and the nighthawk is the only internet connection I have.

I then downloaded the nighthawk appto my andriod phone, and it startsbriefly, but then will not run. Delected, reinstall app, same result.

Any ideas will be welcome for this senior citizen with limited tech skills!