@matekaneve 

All the devices can detect the 5GHz but unable to connect to it? Is there any error message? 

Try to change a new wireless network name of 5GHz  to give it a go.

Besides, what kind of channel you have used? Try to use different ones and verify whether it works.

And you can forget the network on your mobile phone or laptop and reconnect to 5GHz.

Please ensure the firmware is up to date; still the same, I'm afraid that you need give it a hard reset and try to connect the default 5GHz.

May it help and have a nice day.

Thank you for your reply, yes all devices can detect the 5ghz network and none of them can connect. I tried everything already you mentioned and nothing worked unfortunately. I ended up rebooting the router and that seems to do the trick. All the devices connected immediately after the reboot. I leave it as it is just now and see if it happens again. If it does, it goes back to the shop for a refund. Thanks again for your help!

@matekaneve 

The same thing exactly happens to me too (5GH drops, 2.4GH works). It happens every once in a while. I selected the option to automatically reboot the router every 3 days, and this reduces the amount of times this thing happens (but this is a weird option: who designes a router that needs to be rebooted every 3 days??).

I have the same firmware/version as yours.

This bug needs to be fixed by TP-link.

@MrZinka Yes, this is absolutely ridiculous. I found a BETA firmware on another website that I flashed on the router hoping it's fixing this issue however I didn't have the chance to try as I put my old modem back in place and I don't want to start playing around and test everything. If you want HERE is a link to the post where they posted a series of beta firmwares for the AU as well as the EU versions. V1 and V2 are included. I have flashed the EU V2 beta firmware on mine and it went fine, the router booted without any issue, but I never connected to the internet as my old modem was already back in action. If you flash the beta and log in to the user interface, you will see a watermark on the background that this firmware is only for testing, but it shouldn't be a problem. If you give it a try please report back here how it works. :)