Do you have the latest firmware? Cause when I go to the emulator for your router ( http://www.tp-link.com/resources/simulator/Archer%20C7_2.0/Index.htm) it shows a USB Settings -> Media Server -> and...

Here I see a big difference, distance is around ~7m, but there are 2 walls between router and that specific place.

Yes, I have a C20i too and it's normal to have weaker signal on 5Ghz because higher frequencies are attenuated more easily by walls.

Check in USB Settings - > Media Server there is a Scan button, but for fixing this in the future so you won't have to click that button, you can set an Auto scan by checking that box and select the...

Did you added in Address Reservation a specific IP for your MAC Address? Most likely the lease time is set to 45 minutes and is releasing the IP and tries to get another one.

Yeah from a wired connected pc to a wireless phone. I've rebooted my phone (S2 Plus) and now seems to run at 32-36Mbps, a little better. Also I had the chance to test it on a newer tablet and it goes...