Cannot connect to EX3700 (intermittent issue)

Hi all, 

I've been having issues on and off with my EX3700 wifi extender since I purchased it several months ago. 

Everything is set up and both the 5G and 2G bands appear in my list of available networks most of the time, however I keep having issues when trying to actually connect.

The problem seems to present itself in different ways intermittently, so I'll just outline each below...

Sometimes when I try to connect, the wifi fan cycles for ages as though it's trying to connect until I'm met with some sort of 'unable to join network' message. Sometimes when I connect I'm prompted to enter the password (which I shouldn't need to) and I'm met with an 'incorrect password' message. Sometimes I'm able to connect to the extender itself, but when I open a browser it is unable to connect to the internet. And sometimes the 2G and 5G bands (either, or both) simply don't show up in the list of available networks.

It's worth noting that the main internet connection in the house is pretty solid and I never really have any issues with my main router, apart from the fact that it is quite weak at the back of the house - but even that signal is more reliable than the extenders' half the time!

Any help or advice is much appreciated.