I have a Comtrend AR-5200u modem and I was having trouble connecting to the internet.
On the IPv6 LAN Application page, do the following:
1) Enable DHCPv6 Server
2) Stateless with Refresh Time of 600 sec
3) Enable RADVD
4) RA interval min of 3 sec
5) RA interval max of 4 sec
6) MTU of 1492
Connect and you should be good to go.