How do I give a vendor access to internal firewall via one of our public IP addresses?

Question

Greetings! 
 We have a network at work spanning three buildings. Two buildings with ELAN's (recently replaced RED-50's) connect to our main building (hub) with a SG-210 UTM. 
 I'd like to give a vendor access to an internal firewall in one of the smaller buildings via one of our public IP's. 
 They need to login periodically to maintain access points and logs. 
 How do I do this? Use a DNAT? Didn't have any luck today. 
 I tried a DNAT from one of our public IP's to the firewall internal IP. 
 Any help would be greatly appreciated.

Jaydeep · Accepted Answer

Hi Sean Rowe 
 This should not be the Service definition unless you want to forward all traffic from the Internet that comes on that specific WAN to the Internal Firewall. Please ask your service provider/vendor to provide details of the specific ports they require.