Okay, I found out that docker was messing with the firewall with all its DOCKER-USER, DOCKER, DOCKER-ISOLATION-STAGE-* iptables chains. I confirmed this by running the rocketchat container in host mode, then by flushing those chains using iptables -F <CHAIN> and switching back to bridge mode.
I added "iptables": false to my /etc/docker/daemon.json config file and restarted docker service in order to prevent it from blocking anything behind my back.
I am glad this problem is resolved, and that the problem is not related to my cloud provider nor rocketchat servers.
Best Regards