Without knowing a bit more about the Airport Extreme's configuration it's difficult to tell.
You may need to add a static route to 176.x.x.x in the 3120 to 10.0.1.203.
You may need to create two VLANs, one for main and one for guest on both the Airport and the Adtran.
By the way, unless you're routing public 176.x.x.x addresses assigned to you from your ISP to guests, you will probably want to renumber your guest network. Private addresses should be in the range 10.X.X.X, 172.16.X.X through 172.31.X.X, or 192.168.X.X. Otherwise you are going to blackhole part of the public Internet to your guest network.