When installing ROS2, multicast needs to be opened as well:

> sudo ufw allow in proto udp to 224.0.0.0/4
> sudo ufw allow in proto udp from 224.0.0.0/4

In /etc/ufw/before.rules, add (before the COMMIT):

# allow IGMP
-A ufw-before-input -p igmp -d 224.0.0.0/4 -j ACCEPT
-A ufw-before-output -p igmp -d 224.0.0.0/4 -j ACCEPT

NOTE:
Disable Ubuntu firewall and it works!
-> check which port the packages enter from