One base multiple robots with unstable network

edit

This is speculative, but you could take a look at a multi-master setup. multimaster_fkie is one example.

Another option is to use ROS 2. It's underlying network stack is better suited for the kind of conditions you describe.

Edit:

I was thinking about multimaster_fkie, is it immune to connection problems?

No, it's not immune. But at least all nodes registered with the local master will be able to continue communicating if/when your robot goes out of reach of your base.

I will read about ROS2 and if I can somehow move all my code to it.

you don't need to. Look into the ros1_bridge node.

Edit2:

ros1_bridge looks promising. As I understand I would to setup ros2core on the base and then connect everything and setup bridges on every robot, right?

Yes, that is one possible setup. It'll probably take some fiddling to get working correctly though.