When connected to either TP on ANY device (laptop, iPhone, iPad) I always have full 3 bars - so I assume full signal... What's the link speed of wireless network connection on your laptop? On web...

Disconnect powerline adapters from the network and try to connect to AirPort Extreme directly. Do they work?

Provide your topology.

Your PA211 is out of warranty? You can buy a TL-PA4010.

The utility for Windows 10 has not been release yet, right? I didn't see it anywhere on TP-LINK website.

You may read some guide but it seems that you didn't realize that your C7 is not working as a router anymore but an access point... In this case, to control internet connection, you should configure...