I am trying to disable auto-negotiation on the WAN port of my Asus RT-AC66U router. This is because it has problems with the CAT5 cable which is embedded inside the wall of my apartment. The issue is that on boot, sometimes it recognizes 1000BaseT, and sometimes 100BaseT speed, and I have a 300Mbit contract. This cable is cross wired (from provider switch to my router), and right now it cannot be replaced. I have tried with Tomato and Merlin firmware, and I don't have this option to modify. Should I buy a cheap/used Cisco router? I only need 1 Wan and max 5 LAN outputs. And what would you recommend.