I just got off the chat with them today about this.  My 2.4 band continually drops out and I have rebooted, reset, change channels and width and it still drops out.  I told them about all the different models of TP-Link dropping the 2.4 band and all they said was it's out of warranty so they cannot help.  I know it's out of warranty but with all the same exact problems, they should know what's really going on and fix it.

Well, it's off to another brand, any ideas?  

I sent mine for repair. Here's what they found:

DAMAGED WI-FI SIGNAL INTERFACE

WI-FI SIGNAL CHIP SET CONTROLLER DAMAGED

SOLUTION: NO REPAIR

TP-Link never again!

@JoaoMachado 

This is why I didn't bother sending mine in. I got mine directly exchanged by Amazon after explaining the situation. However, sadly, Amazon replaced it with another C5400, which has the exact same issues. I honestly think there's a fatal flaw with these devices that cause the 2.4 Ghz chip to heat up and knock out. The longer you have it, the quicker it does it. The 5 Ghz chips don't seem to have this issue. I bet that TP Link knows about it, which is why they released a second C5400 (the C5400X) which is identical feature-wise. Nov 2018 to March 2019 was my initial go, and so March 2019 to Jan 2021 and it's finally off my network because of some more screwiness (won't surf to certain websites, even after turning off homecare). 

Anyway, good luck. 

My advice if you're going to ever buy TP Link - check the product forum like this one for issues before you buy. Do not trust the reviews, blogs, etc. Testing a device for 1 or 2 weeks isn't going to give you a true picture of how the device performs.

Add me to the list  of 2.4Ghz disappearing. I had my 5400 replaced with another 5400 and the problem is now worst. With the old unit, it happened maybe once in 7-10 days but with the new one, it happens every 3-4 days sometimes twice in 2 days. The router is fast when it's working.

What's everyone going with beside TP-Link?