You didn't give us much info to go on, public IP / port / internal network settings...So -
A shot in the dark for you. 5 IP addresses...there is no netmask for that, i.e. /30 has 4, a /29 has 8 a /28 has 16 ip addresses - how are you getting 5?
Ip addresses aside - I am going to assume you are using winbox to interface w/your ROS
Go to Ip / Firewall / NAT. First add a new rule, dest-nat - the Public IP you want to forward ports from. Action tab - the private IP you want those forwarded ports from the Public IP to go to. Select the ports. You might also want to select the dst port on the first tab as well, i.e. port 80 for http traffic - that way no matter what port the request comes in on as long as it is destined for port 80 it will get forwarded.... Now 'kind of' the reverse for src-natting from your private IP address / port to the Public IP address / port - same as before except you select scr-nat as the function instead of dst-nat....