First, you don't need anything that is DOCISS 3.1 compatible. That is the whole point of going fiber(FTTH). Cox provides a optical network terminal(ONT) for free(stays on the property). See here for connecting your router to it. The only requirements is it has a WAN ethernet port and can handle DHCP. The only exception to this is RFoG which is different then true fiber. You can tell by the upload speed. DOCSIS currently caps at 100Mbps upload. Anything faster then that is fiber.
I am a fan of Asus because I like their user interface and features. At that price point I would suggest the RT-AX58u. Here is a review of it.