AT&T Internet uses its own residential gateway that isn’t very powerful or configurable. For a while I used an AT&T U-Verse connection as a failover for my home network, since I work from home and Spectrum outages meant I couldn’t work. So here’s how I configured an Asus dual WAN router with AT&T Internet, using AT&T U-Verse as a secondary connection, with Charter Spectrum on my primary. This will also work the same way with AT&T Fiber.
I switched back from Spectrum to AT&T exclusively. Here’s why. But you may want to set up failover between AT&T and another provider. It worked reasonably well for me, and it’ll probably work at least as well for you, too.
First things first: I assume you have your primary ISP configured and working. Hopefully it’s simpler than AT&T. Charter is pretty easy. You tell them you want just a cable modem and you’re using your own router. Most cable providers will work the same way.
I also assume you have a laptop with a wired network jack you can plug into each ISP, because you’ll have to check some things on both sides.
Configuring your Asus router for dual LAN
Log in to your Asus router. Scroll down to WAN on the side. Click the Dual WAN tab at the top. Under basic config, you’ll need to turn dual WAN on, set the primary WAN to WAN, set the secondary WAN to Ethernet LAN and pick a port, then select Failover mode and check allow fallback.
Under auto network detection, I enabled ping to internet and choose Google from the dropdown.
Click Apply to save the changes.
Navigate to WAN > Internet Connection and scroll down to WAN type. Select Ethernet LAN. Scroll to LAN DNS and choose No for connect to DNS server automatically. Enter 22.214.171.124 for DNS Server 1 and 126.96.36.199 for DNS Server 2. If you don’t hard code your DNS, it will try to use 172.16.0.1, which isn’t going to work.
Click Apply. Now it’s time to configure your AT&T device.
Configuring your AT&T gateway for dual LAN
The gateway that AT&T provides doesn’t have a lot of configuration options and the options it does have aren’t intuitive. It defaults to a 192.168.1.0/24 network and an address of 192.168.1.254. If your Asus router doesn’t live on 192.168.1, you’re golden. If it does, you’ll have to reconfigure the AT&T router so you don’t get a conflict.
If you’re not sure, assume your Asus router does live at 192.168.1.1 and you’ll have to reconfigure on the AT&T side.
Plug a laptop into the AT&T gateway and log in. The password is on the underside of your router. It’s probably just a long number. Navigate to Settings > LAN > DHCP. Change the range to 172.16.0.0.
You’ll lose your connection at this point. This is a pretty good time to plug in your Asus router. Then either unplug your laptop’s network cable and plug it back in, or renew your DHCP lease with the command sequence ipconfig /release followed by ipconfig /renew . Now you’re ready to reconnect. Point your browser at 172.16.0.1 and log back in.
Navigate to Settings > Firewall. Click Applications, pinholes and DMZ. You should see two devices on the screen: your laptop and your Asus router. Click on the Asus router, then click Allow all applications (DMZ plus mode).
Go back to your Asus router. Click on Network Map. You should see your primary WAN as connected and secondary WAN as standby. Click the primary WAN and you should see your public IP address. Click the secondary WAN and you should see a different public IP address. If you see a 172.16 address on your secondary WAN, revisit your settings on the AT&T router and make sure you didn’t forget to click apply somewhere.
Visit IP Chicken to view your IP address. Unplug your cable modem from your router and wait about one minute, then refresh the page. You should see a very different IP address, since you’ll be coming from AT&T now instead of your cable provider. Make sure other common sites work too. Then plug your cable modem back in. Visit IP Chicken again. Your IP address should change again.
That’s it. Now you have a redundant Internet connection from two different ISPs, with automatic failover.
Revisiting some settings
Now that you’ve verified failover works, it’s a good idea to revisit a couple of settings.
Log in to your Asus router again. Scroll down to WAN on the side. Click the Dual WAN tab at the top.
Under auto network detection, I think the default failover may be too high at 60 seconds, and the default 20 seconds for failback is too short. Your mileage may vary.
If you set failover too short, you’ll jump over to your slower connection more often, but that’s better than a long interruption. Experiment with failing over while streaming some video or music and see if you can come up with some settings that let you fail over and back without interrupting it too badly. I ended up maxing out my failover settings because Charter outages would often yo-yo for a while and the router would get confused if it flipped back and forth too many times in a short period. This meant I might stay on AT&T 15 minutes longer than necessary, but I found it worked better for me that way.