How can I connect to a ROS master and the internet via two different networks?

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Note that you'll also need to make sure -- if using hostnames instead of IPs -- that the DNS server you're using knows how to resolve the hostname(s) for your robot(s) to their IPs, or this likely won't work.

At first, thank you for your quick answer!

We already tried that. The command "rostopic list" shows us all published topics from the robot. But when we start a ros-node that subscribes a topic from the robot (e.g. robot/state) we get the following error: OSError: [Errno 110] Failed to get robot state on robot/state.

It is a timeout error, so we do not receive messages from the robot, but the topics are known

The roswtf command tells us there is an unknown host for all the topics from the robot and these noded are unconnected. How can I solve that?

The roswtf command tells us there is an unknown host for all the topics from the robot

that would seem to connect to my comment about "make sure you have a working DNS server if using hostnames".

As a test, you could see whether adding the appropriate information to your /etc/hosts file makes thing work. If it does, you should probably use a local DNS to resolve your robot's hostname, and have it forward queries for all other hosts.

If you're absolutely sure the robot will never ever get a different IP, then using /etc/hosts could be acceptable.

We added the following line to /etc/hosts:
"Ros-master ip address" "hostname given by rostopic info" and it worked.

Thank you!