when you use the gateway, it act more like your computer, so instead of pppoe from your computer, the router will do that for you once you need to connect.

the wireless router will then "routing" for wireless and wired network. it use c-class ip which is 192.168.0.1-255, you need to set the router to enable this routing function.

the router can set up as "gateway" or "router", try both and see if any work.

when you connect the cable to 1-4 port instead the "uplink" port, it just route the ip through.. so the 3com is not do anything.. it just pass through the signal.. so you can get online. but can't "routing"