How to set up second IIS server in SBS environment?

We run SBS 2011 at the heart of our company with Exchange, AD, Sharepoint, DNS, DHCP, and IIS all running on the main SBS server. We also have the premium license running on a separate server which hosts our ERP. The ERP vendor just added a web portal to access the database which runs on IIS on the second server. I already have an A-record added to our web hosting company and this same FQDN bound to the website in IIS on the ERP machine. IIS is not my area of expertise, I tried adding just an A-record in the SBS DNS but that only allowed traffic to be routed from inside our LAN. What configuration needs to be done on the central SBS machine to direct traffic web traffic to the ERP machine for this new portal?