I bought a pair to extend network to summerhouse at end of garden - about 70m.

I setup one up initially to test as an access point - standard settings, however it took a few mins for the ssid to appear, however when trying to connect from a phone or laptop, it would not connect successfully.  After four unseccessful attempts on the laptop, the ssid dissapeared from the phone and from the laptop.  Tried a few times after some resets but still the same issue.

After testing lots of scenarios, extension cables and even a wireless doorbell pass through socket - which plugs into a power socket, I found the only way it was worked succesfully was for the poe injector to to be plugged into the wireless doorbell passthru power socket.  So the wireless doorbell must be some sort of filter???

ELsewhere in my home network, I have a Tenda Powerline AV1000 extender kit - so I turned that off. Note that the CPE was not plugegd intoyhis kit during any testing  Bingo -that was the problem - so issue was the existence of the powerline stopping the wifi ssid being connected too.  Really bizarre even with all the cpe 510 indicator lights looking good,

So did the powerline kit interfere with the poe power  device supplied by TPLink? Looks like it did.  Took about 8 hrs to resolve - the wireless doorbell gave me the clue.  Hope ths will help someone.