Thank you for responding Pat. I'll start with some background.
I have a 4 port NIC assigned as WAN1, WAN2, OPT1, and OPT2.
OPT1 and OPT2 are in a LAN bridge in case I needed another LAN port for whatever reason.
0_1550730710630_Interface Assignments.PNG
0_1550730780799_Bridges.PNG
Here are the interfaces in my dashboard.
0_1550731230613_Interfaces Edit.png
To answer your first two questions, both WANs are seen as a DHCP connection as shown.
0_1550730977974_Gateways Edit.png
This is the gateway group I have.
0_1550731101330_Gateway Group.PNG
The firewall rule for the LANBRIDGE interface (my LAN) is set to use the gateway group I named "Failover".
0_1550731055893_Firewall Rule.PNG This is where I think the problem may lie but I'm not sure.
Here are some of the thresholds for Gateway 1 (Comcast).
0_1550731185009_Gateway 1 Thresholds.PNG
Since WAN2 wasn't working as a failover, I instructed the client to simply use the SSID from the AT&T 4G gateway modem so they can have something. I haven't yet put the AT&T into bridge mode yet as there doesn't seem to be a "proper" way to do it. It seems the true WAN IP won't for this 4G modem won't be on the WAN2 interface unless we pay for a static IP, but either way, double-NATing shouldn't be a problem as all the client needs is a simple internet connection to function. If it was working correctly I would have disabled the WIFI on the AT&T device (actually not sure if it will even let me. This thing is pretty locked down.) Please let me know what you think of if there are any other pieces of information that would help in solving this issue. Thanks in advance!