Cisco RV320 GW to GW VPN from private IP address

I'm connecting two Cisco RV320 routers together over VPN - an office and a residence. The office gateway has a static public IP; The residence gateway has a dynamic public IP. The purpose of this VPN is to connect an office IP phone that been moved to the residence.

I was able to do this successfully, repeatedly and reliably when both routers are at the head of their respective networks. If I must, I will deliver the router in this configuration. It will require bridging their current AT&T U-Verse router so the RV320 gets the public IP address. It will upset their current network configuration, but will get the job done for this user.

I would like to save this user some trouble by configuring their RV320 to work from BEHIND their U-Verse router. That is to say, the remote gateway WAN1 port will have a private IP address when it reaches out to the office gateway. This turns the RV320 into a network endpoint with only the office IP phone connected. I've had sporadic success creating a tunnel, but I cannot get the tunnel to reconnect if it becomes disconnected. This needs to be seamless and automatic.

Can someone tell me what I'm missing or suggest another course of action that would work better? Thanks for reading.