It is good but reboot what?

Try to fix different channels for main router and the extender. For example, the main router uses channel 1 and extender uses channel 6 or 11.

Umm, can you access internet wirelessly with the correct IP? Since D2 is working as a router, what's the IP/Mask obtained on Status page of D2? Have you done MAC Clone?

SNR Margin and Line Attenuation indicates line quality. If SNR Margin is lower than 10dB or Line Attenuation higher than 45dB, your line quality is poor and may suffer from Internet dropping. In this...

Sounds strange. What's the hardware version and firmware version of your TD-W8970? Are you trying to login the web interface locally after set it up? Have you tried any other web browser or different...

Default Wi-Fi name of RE200 should be TP-LINK_Extender_XXGHz and it is unsecured. Try to reset it by pressing its reset button for 1 second, look for the right name and connect. If you can connect PC...