Im having issues since I got this router and I want to get advice before the time I can return it to the store ends.

My main issue is with the WiFi, for example:

• Some devices wont connect to the 2.4Ghz, but they will connect to the guest 2.4Ghz.
• The internet just stops working on a band. For example, devices connected to the 5Ghz lose internet access and device reports, "Connected, without internet", but the 2.4Ghz band works fine and so does wired.
• Randomly the 5Ghz band just kicks you off and reconnecting doesn't work for a couple of hours.
• Many devices have to reconnect to the Wifi after rebooting.  For example xboxs wont automatically connect to the wifi. It reports a network error and I have to the connection settings on the Xbox and simply press OK on the already filled in wifi password.
• For a couple of hours the router just refused to connect to the internet, however I could connect to the internet by plugging straight in to the router myself. 

I bought this as ex-display so has a limited time warranty.  I do believe it was an exdisplay and not used, as all the plastic peal stickers are stil on it.

I have updated the firmware to the latest on.  I had to do it manually router interface reported it was the latest version but going to the TP-Link support page showed a newer version.  But after updating the firmware it still doesn't work as i'd expect.

Any advice on what could be causing the issue? Any settings I can try?

Many thanks.