Well I can't find any resources for that particular model: 210014
Typical DSL router/modems from ISP are cutdown versions of real routers and I'm not sure any support multiple WAN IPs.
What model is the Westell? B90 is version. Does your router have a web interface to configure it. The B90-210015 is a bridge modem meaning it only converts from DSL to ethernet, so its likely your westell is similar
In order to route to your LAN, a router must be configured with an IP. So you'll lose one of your static IPs for routing.
To route you will need a full fledged router. Not a Linksys, D-Link or anything else found at a retail store, think Cisco, etc.
With you current setup set up 1 computer. Since the westell is probably bridged already -if not configure IP Passthrough, assign your PC an IP, ISP DNS, and PPPoE and connect. You should have no problems.