@fhantazm 

Can you detail any specifics about the 2.4GHz having difficulites connecting?  Type of device, model, current OS/Software version?

@Dr_Mono 

any specific types or models of devices?  Is its everything not connecting?  Have you tried to enable the guest network on the 2.4GHz band to rule out a SSID/password conflict?

@Dr_Mono 

Have you attempted to reset the password?  Also try to forget the network on the MAC and then try to reconnect as a fresh connection.  Lastly try the guest network with a different SSID and password and see if that work.  There are no such connection bugs that have been reported though so I don't believe it would be that.  

@Carl 

I have the same issue.

Trying multiple ways to solve including to use a unique SSID and password for 5 GHz wireless network,reset the 2.4 GHz channel from auto to 1,6,11 (as suggested by some of the users),relocate the router,cut off some of my smart devices (Clients) and etc.....BUT to no avail.

Seeking help from TP-Link technical advisor,hopefully it can be solved,really annoying.

@Dr_Ho 

Same router?  What hardware version and What firmware are you using?

@Kevin_Z 

No worries at all, you may be more inline to answer the question as you may have more direct experiance with the product.  Feel free to jump in and help anytime you have anything to add.