AC1750 Access Point Stopped Connecting to Internet

My Netgear R6400 AC1750 router that was configured as an Access Point in my bedroom stopped working.  How I noticed it is when my Firestick could not connect to it. Then I checked my Iphone 8 and my Dell computer and came up with the same result. You can see the network name and and its got full strength, but when you try to connect to it with the phone or the computer, it just shows a circle going round and round and it never connects.

I installed it as an access point at least 3 years ago and have not had any problems with since, until last night. 

What I tried:

a) unplugged the main router for at least a minute and restarted it. Nothing.

b) unplugged the access point and and the TPLink powerline adapter for at least a minute and plugged them back in. Nothing.

c) upgraded the firmware to my Netgear R6400 AC1750 MAIN router. Nothing.

As clarification, both the main router and the access point are the same Netgear R6400 AC1750 units.

I have not upgraded the firmware to the Access Point, first because I haven't researched how to do it and second, I don't know if the main router upgrades the firmware of the Access Point through the Powerline Adapter. I don't even know if that would make a difference.

The wireless signal from the main router is working fine; it's just the Access Point that I cannot connect to.

I would appreciate any suggestions anyone might have.

Thank you.

I would appreciate any solutions