Google Fiber Router Settings
First step is to get to the router page in the GFRG100 router to tweak some settings, then follow these steps:
Step 2: dial *41 from your phone connected to the DUO and write down the IP address you hear.
Step 3: Go to the router setting page and click on the DUO that is listed on the left under "Devices" (it may use the IP address as the name)
Step 4: Click on Advanced Settings for the DUO device
Step 5: it should take you to a page like this: https://support.google.com/fiber/answer/4650342
Step 6: Turn Reserve IP to "ON" and enter the IP address you wrote down in Step 2 (for example, it will be something like this: 192.168.#.#)
Step 7: Turn DMZ to "ON"
Step 8: Add a Port Forwarding Rule: https://support.google.com/fiber/answer/4650342#portfwd
A. My Device = DUO IP (the same IP you previously wrote down)
B. Select a service type = Choose Custom to enter single port
C. Enter the port number 5060 (if it is taken, then chose 5085 or 7060)
D. Select a protocol from the Protocol drop-down = TCP/UDP
E. Finally, click Apply.
I hope this helps!