You shouldn't have a broadcast 255 at the last octet. Anyway, here is what you should do.
1. Get the settings that used to connect you to the DSL line (winipcfg on the Win98 box).
2. Plug those settings into the public interface of the Mitel box. (you need the IP, the gateway, and your DNS server info).
3. Set a static internal address of 192.168.1.1 for the private interface.
4. Setup eSmith to be a DHCP server and setup Win98 client to use DHCP
(or alternative step 4)
4. Setup static IP in Win98 to 192.168.1.2, Gateway 192.168.1.1 and DNS settings.