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. :)