After buying an ArcherC6 (european version, v2) in Amazon, it was working for a couple of weeks at gigabit speeds. But now, for some reason, no matter how many times i reboot it, it always syncs to 100mbps instead of 1000mbps full duplex.

I've tried forcing 1000 mbps in archer's settings, I also forced it on my pc. It does show, however, that it's syncing to 1000mbps with my pc. The problem is between the archer and the main router. 

This is my main router screen, you can see it's negotiating 1000mbps full duplex with my provider, though archer is listed as 100MB FD:

This is archer's network screen, it shows it's negotiating 1000mbps with my pc, though only 100mbps with main router.

And this is the setting I tweaked to no avail:

Lastly, I used a cable tester with the cable that runs from archer's wan port to main router's lan port, and it seems the cable is OK, as the light went through all numbers.

Can you help me to fix it? Thanks in advance.