I'm having the same problem. 

I've tried:

1. Restarting modem (multiple times)

2. Restarting devices

3. Multiple devices

4. Changing channel on modem setttings

5. Resetting the modem and changing the name and passwords

Some have resulted in a temporary fix but nothing permanent.  I'm thinking TP-Link has some major issue and there aren't enough of us complaining for them to do a firmware update for it.

@Luke_B 

there is still no solution to the problem. 2.4 ghz still kept disconnecting. Dont buy tp link product. Made in china is thrash 

Having same issue. Have the archer c2700. Went through support and had to return and sent another one. Worked fine for a month and then same thing just started happening. The 2.4 band is trash. How come they have not had any real answer for this issue?

@Dflynn I've sent mine back had a full refund and bought the Nighthawk and very happy with it. Tp link customer care made my mind up when they said they would get back to me in 24 hours to resolve the issue. 2 weeks later nothing. Shocking customer care.

Same thing here.

5 Ghz 1 and 2 working.

2.4 Ghz not. 

C4000. Is TP-Link trustworthy?

Interesting you all are having this problem on the newer routers. I also just started having this issue on an older N-600 TL-WDR3600. I've had no problems with this router up until the last few months. It has always been very stable both for both WiFi bands and ethernet connection. But my issue with 2.4Ghz presents exactly the same as you all have described. It will be working fine, then suddenly only 2.4Ghz drops. The network cannot be seen but the 5Ghz is there and working and the ethernet is still working. The only resolve is to reset the router, in which case 2.4Ghz comes back on it's own, works for a day or two, then drops again. I figured since I have been using this router for maybe coming on close to 10 years that it was just on its last legs, espcially since it is getting more of  work-out now than ever (work from home, school from home, etc.). I have tried tweaking all the normal settings, and updated to the latest firmware available for this router (2015), to no avail. 

That said, I have been a very satisfied TP-Link customer other than this new issue. But I have some hesitancy to get another now that you all are having this same issue on newer routers. Prior to this WDR3600 I had been using a TP-Link 2.4ghz router so I've been a complaint free customer for probably 13 years now.

My last attempt will be to try and bake the DD-WRT firmware onto this router. Maybe that will cure it and allow me to put the VPN on the box rather than using software on every machine. If that doesn't work though I'll need a new router on hand because I may not be able to undo my handy-work. 

@Danize @Dflynn @JoaoMachado @bluefordredschw @Adin 

I've found a solution to save my 2.4 ghz network: turn off the 5ghz network.

I know it's bullcrap, but I need 2.4 for my older devices that can't run 5ghz.  Plus I want the distance provided by the 2.4.

My previous fixes didn't work.  Thanks a lot TP-Link!

@Luke_B 

Maybe I will try this as well. The 2.4ghz is the most important in my network as well. It'll be some time before I abandon that entirely.

@Luke_B Luke, I tried today without success. 2.4 Ghz is visible and some devices can connect to it with 5 Ghz off, but quality remains poor. I gave up.