Make sure you enable WAN2 for internet connectivity, also disable WAN1.
Also once everything is set up power off the router and modem for a minute, turn on the modem and let it sync fully (give it 2 minutes) and then power on the router.
BTW The 2820n only has a maxiumum throughput between the two wan links and the lan of 50Mbit/sec so you may find you never achieve full speed from it.