Sonic doesn't give you their own router in addition to AT&T's. You just get AT&T ONT and CPE.
You have some options.
- Use your BGW210 as your primary router and connect all your devices to it. That's the default.
- Connect your own router and set the device as IP-Passthrough and the BGW210 will "bridge" your WAN address to your router's WAN port. Connect all your devices downstream to your own router. If you perform a traceroute, you should notice your packets will pass to your router and then to the BGW210 before it leaves your home. This is the "double-NAT" most people talk about. I don't know what the CPE is doing but it's sort of a hybrid double-NAT. Port forwards will work correctly but all your packets will go through the BGW210's anemic 8K sessions table. Create more than the sessions limit and you will no longer be able to use the internet until other sessions are closed.
- Replace AT&T's CPE with a pfSense- or Ubiquiti-based firewall. There are several ways to do it but the first is to obtain a certificate from an AT&T router. As these certificates are unique to each device, many purchase one from eBay, root it, then import into the firewall. Look through the U-verse forums at dslreports.com
I went with the second option but instead of the BGW210 which I received, I purchased a Pace 5268ac and use DMZ-Plus. The 5268ac doesn't have the double-NAT behavior and its sessions table is twice as large, although I've found that its sessions table doesn't limit my connection. The 5268ac has a really slow webUI but since I hardly use it, it's not a problem. I also disable the CPE's WiFI because I have my own.
I'm still hopeful Sonic will eventually build their fiber network here in the East Bay and I don't want to bother with using pfSense/Ubiquiti. Sonic-based CPEs have true bridge mode and hopefully at that time I will also have the option of returning it.