After struggling a bit, this is now setup correctly. Advanced Comcast support helped with fine tuning the setup. However, I spent a lot of fruitless hours with two first line Comcast support personnel who were following some scripts and did not have a full understanding. The third one I spoke to was smart enough to transfer me over to the advanced support. The advanced support person (Ameira) was very knowledgeable and provided excellent help.
I have
1. Apple Airport Extreme (flat round rectangle shape - don't know which gen).
2. Xfinity Modem with WiFi - vertical one.
3. Vonage modem for landline
4. Network printer
Xfinity Modem Setup
1. Set Xfinity Model in Bridge Mode (not DHCP). This also disables its WiFi thus avoiding a duplicate WiFi network in case you don't want to use the WiFi network that comes with Xfinity modem.
You can either have the Comcast rep turn on Bridge mode remotely or connect locally to the router on the address 10.0.0.1 (default username / password is admin / password)
2. Connect Apple Extreme using Ethernet cable to the Xfinity modem in network jack #1. Not sure if #1 is critical but thats what Comcast support recommended.
Now, VERY IMPORTANT, don't use any of the other network ports on the Xfinity modem. Remember, its in Bridge mode and its not acting as a router anymore.
I had to figure this out myself because I had connected my Macbook using a wire directly to the modem to get access to the internet for troubleshooting not realizing that this was causing the Airport Extreme to not establish an Internet connection using the Bridged modem.
Airport Extreme Setup
1. Using Airport Utility on your Mac, connect to the Airport Extreme.
2. On the 'Internet' tab, set to DHCP. No need to fill in any of the other fields.
3. IMPORTANT -> On the 'Network' tab set to "DHCP and NAT". I had to change the NAT address range to use the 192.168. prefix instead of 10.0. Not sure of the reason for this.
4. Create your WiFi network.
After enabling both 2.4 and 5 GHZ and measuring download speeds, I found that when both are enabled 5 GHZ works faster. However, 2.4 GHZ only (i.e. disable 5 GHZ) works fastest - faster than 5 GHZ. So my setup is 2.4 GHZ enabled, 5 GHZ disabled.
5. Connect Vonage, Printer by ethernet cables to Airport Extreme.
Finally
Power cycle the Xfinity Modem, then power cycle the Airport Extreme.
