I can agree with FurryNutz's suggestion of turning off Media Prioritisation, the second post on this thread
I upgraded to Fibre and had great speeds here in UK of 37Mbps download when the install engineer connected directly to the modem, but between 3 - 7Mbps via the EA6700.
Spent several days tryng different things, including nearly 3 hours on Linksys Chat yesterday where all we done was reset my router to factory defaults, manually update the firmware (was already updated, but hey, might as well go with it) and changed the wireless channels. Didn't make one drop of difference.
But later I read on this forum about Media Prioritisation. 'Resetting' the Media Prioritisation and then turning it off, speed went up to 23Mbps immediately and later back to full 37Mbps speed.
HTH