Link Aggregation? What have you done? Did you bind two network cards on your PC?

Using the same PC, connect directly to your modem and test if you can download properly. Disable ESET and test. When browsing webpages by the router, there is no problem at all? Try to upgrade the...

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...