VPN use case for ROS_IP, ROS_MASTER_URI and ROS_HOSTNAME

edit

I am not sure whether being over a VPN is relevant but here's my scenario:

1. Robot running ROS Melodic, publishing /odom and subscribing to /cmd_vel

2. Cloud based docker environment running ROS Melodic

3. Both the robot and the cloud environment use tail scale to create a vpn so that they can talk to each other over tcp/ip. Call robot's IP vpn_robot and cloud environment IP vpn_cloud

4. Able to ping in bother directions between vpn_robot and vpn_cloud

5. Issuing cmd_vel using rostopic pub from robot to robot works fine

6. Viewing odom on cloud environment using rostopic echo works fine

7. roswtf gives no errors on both sides

8. on Robot, ROS_IP and ROS_MASTER_URI both reference vpn_robot

9. In cloud environment, ROS_IP references vpn_cloud and ROS_MASTER_URI references vpn_robot

10. Roscore is running on the robot

But...

Sending cmd_vel from cloud environment to robot using rostopic pub DOES NOT WORK! I tried defining ROS_HOSTNAME equal to ROS_IP and that made no difference.

I am looking for suggestions on how to further troubleshoot this. Thanks!