Add a new rule at mangle with action accept and set destination IP and put those rules at top of others!
Those are mine:
/ip firewall mangle
add action=accept chain=prerouting dst-address=192.168.1.0/24 src-address=192.168.1.0/24
add action=accept chain=prerouting dst-address=public-ip-of-website
add action=mark-routing chain=prerouting new-routing-mark=to-vpn passthrough=yes src-address=192.168.1.0/24