Same here.  All was fine for a few weeks.  I can have up to 36 @ 2.4Ghz smart home devices, cameras and computers connected. Was working great no drops for days then last night I went to stream NetFlix in the spareroom on a Visio TV and it connected to 2.4Ghz antena.  This was a first time using this connection and right off hitting play I could see that was not going to work. I switched to another slave router (Almond) and was able to watch with no issues or so I thought.  All the WiFi devices on the AC5400 2.4 were disconnected and unable to reconnect.  Alexa's and divices on the 5Ghz were still connected but the Echos 2.4Ghz were not able to reconnect.  Going to the router menu the responce was painfully slow to switch pages and some just failed and it was clear all 2.4 devices were not there.  I attempted to disconnect the WiFi using the face button but the light status did not change. It looked as if the router was having a software logic issue.  I powered down unpluged then powered back up this router and that seemed to help.  I made sure the TV was off and went to bed. In the morning I woke up to see all 2.4 were disconnected also using the router pages the couple 5 were connected and none of the  30+ 2.4 were talking.The slow responce and even hanging pages were back. So this time I just used the switch in the back to power down and back up.  I now worry this is going to be a reoccuring issue with load ballincing. In conclusion it really feels like a logic issue with that antena(s) and or the processor that handles the web interface?  I can keep you posted and will do a factory reset only if nessassary as a last result. In closing with smart homes and older apliances (TV, comuter, gaming system) all wanting 2.4Ghz some industry disclosure of max capasity would have been helpful. With only a couple weeks on this router all seened great as it replaced a DLink DIR-890L AC3200 but maybe I did not take a high enought step up?  I'm wishing I knew about your newest just released TP-Link AX6000 quad core X capasity but here again would it solve my older smart home 2.4Ghz issue?  

Firmware is up to date as of 12/2018:  1.2.2 Build 20170912 rel.56240(5553) / ArcherC5400 v2.0

Quick question are the 2.4Ghz antenas (guest/non-guest) sharing the same CPU, ofloading to quest would have no benefit? Perhaps reprogramming ne of the two 5Ghz wireless antenas and dedicate that to a seperate 2.4 channel thus giving a backward compatable (non-gamer) home duel/quad 2.4Ghz with just singlle 5Ghz.  Then later on step back up when perferals adopt the new shorter length higher frequency. 5Ghz band. 

My first router after 6 months started dropping the 2.4 band. Both 5G bands stay connected.  Got a replacement.  Replacement has lasted about a year and is now doing the same thing again.  Only way I can get the 2.4 band back up and running is turning it on and off or rebooting the router.  I have 77 days left on warranty but at this point I think I'm ready to give up and buy a different brand router.  I was thinking i have too many device hookup up to the 2.4 band a total of 24 but it seems like others are having the same issues with no work around.  I have tried channel 1, 6, 11.  I have tired guest network and nope still goes out but not as much.  When i log into router the 2.4 band won't show any encryption and you can't change it.   Has anyone found a fix?  Is a new router the way to go.  PLEASE HELP.  I don't want to spend $200 plus on another router if I don't have too even though it's been 2 years with the archer.  Thanks  

It is a big shame that such an expensive router didn't receive a new firmware version ever since 2017/9/12. Especially looking at all the problems with the wifi channels. Then, the maturity and quality of the replies (if any) by TP Link 'team' is the worst I have ever met. I am still having the problem that the second 5 ghz is disappearing . Or is there already a workaround about this ISSUE?

I also have this router and I have some serius issues with it!!!

5GHz signal keeps dissapearing, but the most annoying problem is Led controller error. each time it appears in log my connection to internet breaks and for most times I have to restart rouetr to get it connected. I dont know what is the problem and didnt find the solution. It is very bad policy for TP link not to have new update to this router in nearly two years. I will hold my nerves and use this for couple of weeks and will never buy anything from tp link (long time user and i recomended few dozens of their AP or routers to friends but never again)

if someone knows solution please respond or recomend some other router

thx

BYe

@AppieK1 

Firmware updates are only usually released when ther are enhancements or bugs in the software.  Some isuses may be hardware related and as such a firmware update may not be needed.  Also it could be that the version of that device has been deemed end of life.  When this happens firmware update are only made when crtitical issues are detected.  Again this is only if these issues are software related.  If your device is on the latest FW version and an channel keeps disappearing you might have defective hardware and if you are with in the warranty period a replacement can be issued.  You would want to contact support via Chat, Phone or email though so they can collect infromation about your product, verifiy warranty eligiblity and troubleshoot if nessasary.    US support can be reached here:  https://www.tp-link.com/us/support/contact-technical-support/