I recently had Sonic Fusion/AT&T Fiber installed and had double NAT issues with my Ubiquiti Dream Machine and also a Synology Disk Station running Hyper Backup Vault and rsync. Here's how I solved it including the steps to put the gateway in pass-through mode. There are steps here not described in the material Sonic provided me and may be specific to my mix of equipment:
Ubiquiti Dream Machine (UDM and UDM Pro) and AT&T Uverse BGW320-500/BGW210-700 Gateways. These are referenced as UDM/P and BGW respectively.
1) Get the BGW up and running. You should already be there if your AT&T network is installed. I'm assuming the default settings in the below instructions.
2). Plug the UDM/P into the BGW via an Ethernet cable. I used the blue port on the BGW, but any of the yellow ones should do. You have to use the WAN port (blue) on the UDM/P. Do yourself a favor and don't plug in any other ethernet cables to devices or switches as this will make step 4 much easier.
3) Login to the BGW's control panel by going to 192.168.1.254. You will need the BGW access code, which is on the back of the device. This is not the same as the Wifi password.
4) In the Firewall settings, turn off Packet Filters and all the Advanced Firewall settings as it's likely you'll be using the settings within the UDM/P for all this. While you are there, go to IP Passthrough and change the allocation mode to "Passthrough", the Passthrough mode to "DHCPS-fixed".
This is the first point to pay close attention to. Under the Passthrough Fixed MAC Address, use the drop down menu "Device List" to find that the MAC address of your router will have been modified by the BGW. It will likely look like the MAC address of your router with the last 2 characters altered. Use this one! Save the changes.
5) Navigate to the Home Network | Subnets & DHCP page on the server. Because the UDM/P really wants to own the 192.168.1.0/24 network, you have to change the BGW to a different one. Change the Device IPv4 Address to 192.168.0.254, the DHCPv4 Start Address to 192.168.0.64, and the DHCPv4 End Address to 192.168.0.253. Save these changes.
Turn off IPv6 (this was necessary for Synology DiskStation to not see 2 routers. It was necessary to get Hyper Backup Vault and rsync to work. Perhaps other networking equipment won't like IPv6 turned on either). Save the changes.
Once done, I could not get to the new IP for the BGW320-500 unless directly connected via Ethernet to the BGW. This was not a problem on the BGW210-700.
6) Restart the UDM/P and the BGW. Once you do these things you should be able to get up and running. Enjoy.
Ubiquiti Dream Machine (UDM and UDM Pro) and AT&T Uverse BGW320-500/BGW210-700 Gateways. These are referenced as UDM/P and BGW respectively.
1) Get the BGW up and running. You should already be there if your AT&T network is installed. I'm assuming the default settings in the below instructions.
2). Plug the UDM/P into the BGW via an Ethernet cable. I used the blue port on the BGW, but any of the yellow ones should do. You have to use the WAN port (blue) on the UDM/P. Do yourself a favor and don't plug in any other ethernet cables to devices or switches as this will make step 4 much easier.
3) Login to the BGW's control panel by going to 192.168.1.254. You will need the BGW access code, which is on the back of the device. This is not the same as the Wifi password.
4) In the Firewall settings, turn off Packet Filters and all the Advanced Firewall settings as it's likely you'll be using the settings within the UDM/P for all this. While you are there, go to IP Passthrough and change the allocation mode to "Passthrough", the Passthrough mode to "DHCPS-fixed".
This is the first point to pay close attention to. Under the Passthrough Fixed MAC Address, use the drop down menu "Device List" to find that the MAC address of your router will have been modified by the BGW. It will likely look like the MAC address of your router with the last 2 characters altered. Use this one! Save the changes.
5) Navigate to the Home Network | Subnets & DHCP page on the server. Because the UDM/P really wants to own the 192.168.1.0/24 network, you have to change the BGW to a different one. Change the Device IPv4 Address to 192.168.0.254, the DHCPv4 Start Address to 192.168.0.64, and the DHCPv4 End Address to 192.168.0.253. Save these changes.
Turn off IPv6 (this was necessary for Synology DiskStation to not see 2 routers. It was necessary to get Hyper Backup Vault and rsync to work. Perhaps other networking equipment won't like IPv6 turned on either). Save the changes.
Once done, I could not get to the new IP for the BGW320-500 unless directly connected via Ethernet to the BGW. This was not a problem on the BGW210-700.
6) Restart the UDM/P and the BGW. Once you do these things you should be able to get up and running. Enjoy.