I have the same here with a C2600. It's been working fine for years and start doing this since a few weeks. 5g is still accessible so I can connect to the admin page. What I see while the router is in this state is that the channel selection has switched to Auto and current channel is 149. Impossible to change back the channel, the settings dropdown is empty. 

I just created a new thread as I'm having the same issue but on the second 5ghz band.

@shaywood 

Can you share the beta firmware?

@Jerryb 

you guys think that's weird

have you even go to the admin page when the 2.4ghz stops working???

IT DOESN'T EVEN SHOW UP ANYMORE!!!!

check this image out, where the HELL IS my 2.4ghz network ?? ITS GONE!!!

chatted w/ CSR and they claim there "is no beta"

@ionizer5 

That is odd.

I've given up on this router. Its going back and I've replaced it with the Asus AX11000.

@ionizer5 

Mine does the same.

ionizer5 wrote

check this image out, where the HELL IS my 2.4ghz network ?? ITS GONE!!!

 

chatted w/ CSR and they claim there "is no beta"

@BBena 

This thread is about the ax11000, maybe youll get lucky and not have this issue.  Im ready to throw it in the trash.  It wont be replaced with another tp-link.

@shaywood I asked on a number of occasions for this BETA firmware for the AX11000 but tech support denies there is such a thing out there.

Do you mind telling me (us) what version did they give you?   I am also having some 2.4Ghz issues with some of my equipment ...

Thanks! 

Just wanted to add that I'm having the exact same problem with the Ax11000. Every couple of days all my wifi devices are unable to connect to the network. Ethernet works fine. 
 

On my Mac, the wifi tries to connect for a while, then asks for a password (which is obviously normally saved), which I type in, then it times out trying to connect.