@Snerd,

Sorry to hear you're having so many troubles.

REs are infamous for having problems during setup, so you may try to set up the device using a different method than when you first tried, such as WPS, Tether, or through the WebUI?

There should be additional settings available so I would recommend doing a factory reset when setting it up again. I would also recommend directly connecting to the RE and using the web interface to configure the RE to extend your existing network - this way you can be sure that the RE has received the correct credentials.

What router are you trying to connect the RE to? If it is a TP-Link Router, you may try to configure the RE as a OneMesh Extender.

Lastly, I would recommend trying to move the RE closer to the router while troubleshooting so that you can be assured that it is well within the range of both bands.

  @Snerd,

By Direct I mean to connect to the WebUI of the Range Extender. This can be done wirelessly, but I recommend using a wired connection to ensure that you are connecting to the right RE and that the connection will be stable for a firmware update. If you look at Method 2 in this FAQ: How to Upgrade the Firmware of a TP-Link Range Extender there is a set of instructions on how to download firmware from the site and update the firmware manually. You should also be able to see the system logs of the Range Extender, which will be able to provide us a much better idea of what the cause may be.

Since you can see the RE in your list of connected devices, you may try setting an IP reservation for the device to see if this will prevent the disconnects from occurring. Lastly, do you have any other REs connected to your network? If so, you may try disconnecting them to see if the behavior stops.