@Bjaerven that was cause the tech support cited. They did not expressly state not to update.

but having experienced what i experienced and the hassle and time wasted waiting for the replacement, id rather not update my replaced device. 

But if there is a new firmware that will be released say in 2020 id give it a chance. Ill not use the firmwares released in 2019.

I've also suffered from that issue, what helped in my case is do not override default dhcp settings, I wnated network to be 10.0... And I found that tbis in my case caused that issue, os I hard reset router, didn't restor ot feom backup but did fresh ne2 configuration and didn't touched DHCP settings, leaved it 192.168.. And the problem is gone, long time ago. Talked a lot with support about this phenomenon but didn't get any meaningful answer from them. 

@SlavaGo 

Thx for sharing your experience. I didn´t mess with my "192.168...." and still the problem appeared though. I´ll install my replacement device soon and hope that I was only unlucky with the original one. Time will tell.... Cheers.

@slavag I'm also facing the same issue. I've installed the router AC4000, on the first day it was good, but from the second day onwards I could only see the 5G network from  my cellphone. In my laptop I could see the 2.4 G network but couldn't connect. I'm very disapointed with t-plink.

I've had the exact same issue. 2.4 GHz network is disappearing. Tech Support emailed me and said to change the encryption to TKIP for the networks. I got a text from my wife who is home and said the Google Hub display is not connected to the network. I do not believe that this will fix the issue. Any help would be greatly appreciated.

@cy9394 I also have the same setup as you running my fios modem like you. Are you saying just to unplug the power on the fios modem and tp link router and the issue will be resolved? Any help would be greatly aprreciated. Thanks.

@harshakavuri 

I just wanted to share my recent C5400 experience. Purchased from amazon and 8 months later one of the 5ghz bands would not hold any speed and the only way to fix it would be to do a hard reset. It would work for a couple of days and then go back to not working, sound familiar. Anyway long story short TP Link's technical support via, chat, email, and remote access throughly analyzed the router and if you have the patience they will replace the router. I just recieved the replacement which is a C5400X, it's a gaming router and it's a beast. They must have had a bad batch of the C5400 cause I'm not the only one who got the upgrade. Good luck with everything 

@wolfpacker 

I'm on RMA number four, hopefully. This is C5400 router number three.  Original router was replaced on RMA, but it, router two, had a loose screw inside that was an electrical hazard. I asked for a new in box replacement and was sent router three which now has intermittent 2.4 Ghz connectivity and no 5 Ghz ch1 radio. I also cannot connect to 5Ghz ch2 even though it can be seen.

Right now I'm back to usin my old Netgear router. 

@wolfpacker 

I have the same issue.