Hi Fenyo,
unfortunalty the setup, either done by cli or WebGui, does make kind of a fuss to the firewall. You need to reconfigure the UnBoundDNS-Section of the opnSense after you ran linuxmuster-setup (see Murks nach linuxmuster-setup im Setup der opnSense - #18 von Adrian). Otherwise you will probably experiance DNS-Loop-Issues.
Kind regards
Thomas