I just got my MT-6000 and though the default software and OpenWRT works fine, my WAN / PPPoE internet connection drops off every so often for a minute or two.
it is really annoying. My other openwrt router works fine but it seems the issue is with the MT-6000.
I've seen others with similar issues but not any definitive fix.
the WAN interface seems to down/up with 3-4 times from the syslog, probably it is compatibility issue of the Ethernet speed rate negotiation. Please can try these in the SSH of the router:
ifconfig
//Confirm which one interface is the WAN, say if the eth1 is the WAN and fixed 1000Mbps:
ethtool -s eth1 autoneg off
ethtool -s eth1 speed 1000