I'm having a very weird issue with this TP-Link Archer C5400 v2.0 (Firmware 1.2.2 Build 20170912 rel.56240(5553)) router. 

After a couple of hours of normal use, the router kills any connectivity (LAN and Internet) when using the second 5ghz band. It will still show up but it will reject any connections (apple devices will even complain that the "Entered password is incorrect."). When this happens I have to switch to the other 5Ghz band connection and turn off and on that second band. Then it will stay fine for a couple more hours, and back to the problem. 

I've tried everything. Hard reset, different configs, made sure QoS wasn't messing with anything. I even made sure the Time settings were correct with the proper local ntp servers. 

Nothing. The problem keeps coming back and I'm out of options. It definitely looks like a bug somewhere because turning the wifi setting for that band off and on, and hitting save, makes everything work again for a couple more hours.

I don't know if this problem existed before or not in my router, because I used to have a very stable wired connection, so never really used the wifi. Now I'm in a different house (and different country) and wired connection is not an option.  What strikes me as odd is that this behaviour doesn't happen on the other 5Ghz band or the 2.4Ghz one.

Another thing I've noticed (which is probably because I'm pretty ignorant in this area) is that the first 5GHz band gives speeds of 150 Mbps / 50 Mbps whereas if I'm in the second 5Ghz band I'll get speeds of 500 Mbps / 50 Mbps (which is my provider's speed, so that's the expected result). 

Any help or pointers are much appreciated. 

Thanks!