rViz not displaying remote topic

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I am working with two devices (comp1, comp2), both with raspbian stretch including ROS kinetic installed (according to http://wiki.ros.org/ROS/NetworkSetup and http://wiki.ros.org/ROS/Tutorials/Mul... )

On comp1 I have a simple node, publishing a volume (with the visualization_msgs package) with the sample Marker code ( http://wiki.ros.org/rviz/Tutorials/Ma... -> using_markers basic_shapes).

I want to display these shapes on comp2 in rViz. I correctly find the topic (/visualization_marker) and I can make rViz subscribe to that topic. I also set the base frame to /my_frame as indicated in the tutorial.

The weired thing is:

• If I run rostopic echo /visualization_marker on comp1, I can see new messages arriving (therefore I concluded that the firewall and the node itself are correctly setup).
• If I run the node using_markers basic_shapes on comp2, rViz shows all the shapes correctly (therefore I concluded that rViz is configured correctly).

But I cannot think of what could possibly fail - especially since I thought in ROS it would not really matter if a node is executed on the same or on a different machine (as long as the firewall is ok -> what I concluded that it is, because rostopic echo /visualization_markers is showing the correct data being transmitted from comp1 to comp2.

Could anyone help me what could still be wrongly configured - given all the things which are working? Thank you in advance!