Can' t route past default gateway

I had a strange issue this weekend with one of my FGT-60Bs. it is connected to a cable modem through the WAN1 port and has a 3G wireless card failover. It failed over to the wireless card- so we thought the cable provider was down. After various troubleshooting steps, we were able to figure this out... From inside we can ping our default gateway. From outside, we could ping the default gateway. Could not ping anything past the gateway from inside or to us from outside. The internet was working fine through the 3G card. (outside tests involved remote controlling a computer at a remote site not connected through a VPN) The cable provider did a few things on their end, but still nothing. Eventually we plugged a computer directly to the modem and it worked perfectly. Switched the connection from the WAN1 to WAN2 and all of a sudden it works fine... (obviously after configuring the default gateway and firewall policies to use WAN2) Is this indicative of a bad port somehow? i' m finding it hard to believe that we could ping out but not get routing. i' d expect that a dead interface would just not work no matter what. but also that if we had something wrong in the configuration that it would also be wrong when we set up the WAN2... any ideas? Thanks