There is an instruction. You may refer to it and check your configuration. http://www.tp-link.com/en/faq-923.html

Forget the Wi-Fi network on your S4 and S6. Scan for it again and check if it helps. Login web interface of WPA4220 and fix a different channel as your router's. http://www.tp-link.com/en/faq-398.html

Firmware of your D2 is up to date? http://www.tp-link.com/en/download/Archer-D2_V1.html#Firmware Upgrade the latest firmware, reset and reconfigure it. If the problem continues, ask for a replacement...

Why do you disable NAT on PPPoE connection? TD-W9980 is working as a wireless router or wireless modem router?

After upgrade the firmware, did you reset it to factory defaults? If you reset it but devices connected to D2 won't obtain IP address automatically, I suggest replacing it under warranty.

How about status of lights on all powerline adapters? Are you sure that all of them have been paired? http://www.tp-link.com/en/faq-258.html Download the new utility here and monitor all of them in...