After installing Virtualmin, you will get a message on Linux Firewall and the Linux IPv6 Firewall:
Warning! It appears that FirewallD is being used to generate your system’s firewall. Maybe you should use the [FirewallD module] instead.
I suggest that you use FirewallD and leave the modules for what is its, unless you are technical enough to reconfigure your firewall, Fail2Ban is no firewall and can be used to dectect certain attack levels and has filters to recognize them…i suggest to use it with your system!