Mon Dec 11, 2017 10:12 pm
Add a second IP to your router's WAN interface - if the modem is connected to ether1, for instance:
/ip address add address=192.168.1.253/24 interface=ether1
next add a srcnat rule to access the modem:
/ip firewall nat add chain=srcnat dst-address=192.168.1.254 action=src-nat to-address=192.168.1.253
Note that if your WAN is using static IP / dhcp (not PPPoE - I don't see a PPPoE interface in your screenshots), that this srcnat rule needs to come before any general-purpose masquerade rule on the WAN interface. Just drag the new rule up the list so that it is sooner in the srcnat chain than the main "access the internet" masquerade rule.
done.