Ok dug into it a bunch more and i'm guessing that my ATT sim card is only provisioned to get a Private IP address on ATT's network. Hence why the SSG5 WAN interface was getting a 10.129.x.x IP address even when the modem was in bridge mode.
Apparently i would need a sim card provisioned on i2Go APN or something that would give me a Public IP on ATT's network.
Based on this info: https://developer.att.com/technical-library/apns/apn-descriptions-and-characteristics