I have a router provided by my ISP, who recommend tp-link.  I bought a starter pack comprising an adapter (TL-PA8010P(UK) version 3.0) and an extender (TL-WPA8631P(UK) version 3.0).  I am unable to find these models in the drop down list above.

I set up the first extender and then the second one in accordance with the instructions.  On the second one, the lights didn't blink as expected so I set it up using the app on my iPhone.  The result was that the router and the two extenders all had the same SSID and password for both bands.  However, when I tried to connect my iPhone to an extender, sitting next to it, the signal dropped out and after a while the iPhone told me I had no internet connection, which was incorrect.  All the appropriate lights on the devices continued to shine steadily.  Before trying to connect to the extender, the iPhone showed two bars for the router and in the top right hand corner of the iPhone screen, and three bars for the extender in the "My Networks" box.

I then reconfigured the extenders to have individual SSID's for both channels.  The result was that, when I was sitting next to an extender, with the extender showing three bars and the router two, connecting to the extender resulted in the signal dropping out immediately.  After about 5 minutes the connection reverted to the router.  When I left the iPhone to select its own Wifi source when I moved next to the extender, it eventually connected to the router.

I'll contact technical support if necessary, but I hope that by giving these details you might be able to publish a fix that will help others too.