The R6300 says it has gigabit on it. Setup correctly with static address, dhcp off and connection from primary router plugged into LAN port?
Go into ISP modem's settings, dissable wifi (if you have not already), and then look for a setting that says "bridge mode" or "modem only mode" if you cant find those then tell me if there is any settings that has the work NAT associated with it.
Once we have modem/router not acting as a router anymore then lets do a speed test from the AC68U from a wired device.
The ISP modem/router is gigabit right?