5GHz WiFi disabled on RT-AC87U

[tRiFFiD]

I realise this is an old router, but it has been working perfectly fine up until 2 days ago. I'm running the last supported version of MerlinWRT and suddenly the 5GHz WiFi network has just disabled itself.
I did a factory reset last night and it didn't make any difference, so then I flashed DD-WRT onto it and the 5GHZ WiFi was working.

Not wanting to keep DD-WRT, I flashed Merlin back on and briefly the 5GHz worked again. When I say briefly, I mean for a few minutes... and then it disables itself again.
There's nothing in the system or wireless logs, and I really don't know what the issue is. The router is not hot, in fact it's winter where I am and the room the router in never gets hotter than 18C.

When I click Wireless > General, it comes up with a screen that is not the normal screen. Most of the fields are blank and clicking Apply does nothing. However in those brief moments when the 5GHz was working, it would come up with the proper screen allowing me to modify settings. On the Tools tab, it shows the temperature of the 2.4GHz module but 5GHz displays "Disabled".

At the moment I can't afford to buy a new router, so was hoping to keep this one running for a bit longer. Does anyone have any suggestions as to what I could try to get it working again?

 

[ColinTaylor]

Problems with the 5GHz on that model were quite common. Unlike other Asus routers that one used a separate Quantenna chip to provide 5GHz. It was prone to hardware failures and often didn't initialise itself on startup. One of the common things that sometimes helped to get it to work was to remove the power jack from the back of the router while it was turned on. Leave it "on" for about 20 seconds so that all residual power has drained from the internal components. Then plug the power back in and hope that the 5GHz works.

 

[tRiFFiD]

Problems with the 5GHz on that model were quite common. Unlike other Asus routers that one used a separate Quantenna chip to provide 5GHz. It was prone to hardware failures and often didn't initialise itself on startup. One of the common things that sometimes helped to get it to work was to remove the power jack from the back of the router while it was turned on. Leave it "on" for about 20 seconds so that all residual power has drained from the internal components. Then plug the power back in and hope that the 5GHz works.

Thank you for the reply, I will try this tonight as soon as I get home and report back with the result!

 

[tRiFFiD]

Problems with the 5GHz on that model were quite common. Unlike other Asus routers that one used a separate Quantenna chip to provide 5GHz. It was prone to hardware failures and often didn't initialise itself on startup. One of the common things that sometimes helped to get it to work was to remove the power jack from the back of the router while it was turned on. Leave it "on" for about 20 seconds so that all residual power has drained from the internal components. Then plug the power back in and hope that the 5GHz works.

I followed your advice and it didn't fix the issue, but after doing your steps and another factory reset it is all working perfectly now. It's great to have the 5GHz back, thanks!