ROS 2 talker-listener on 2 machines not working

edit

I just solved the issue. I'm going to drop the answer here.

So ROS 2 Galactic default rmw implementation is Eclipse Cyclone DDS which does not enable multicast by default. Unfortunately I was not able to get Eclipse Cyclone DDS working.

So I changed to eProsima Fast DDS with export RMW_IMPLEMENTATION=rmw_fastrtps_cpp After changing I learned that multicast is not enabled by default in eProsima Fast DDS either... So I needed to create a file named fastrtps-profile.xml with the following content inside:

<?xml version="1.0" encoding="UTF-8" ?>
<profiles xmlns="http://www.eprosima.com/XMLSchemas/fastRTPS_Profiles" >
<transport_descriptors>
    <transport_descriptor>
        <transport_id>CustomUdpTransport</transport_id>
        <type>UDPv4</type>
    </transport_descriptor>
</transport_descriptors>

<participant profile_name="participant_profile" is_default_profile="true">
    <rtps>
        <userTransports>
            <transport_id>CustomUdpTransport</transport_id>
        </userTransports>

        <useBuiltinTransports>false</useBuiltinTransports>
    </rtps>
</participant>
</profiles>

This enabled multicast in eProsima Fast DDS. To make DDS use this configuration file, you do: export FASTRTPS_DEFAULT_PROFILES_FILE=<path_to_config_file_created>.

Do this in both machines and it should work!