Ok, now we see the problem, at least i think so.
When you connect a computer to a modem (no matter what's the media) and you run pppoe in the computer to establish a connection, a special conceptual techonolgy is used and it is named numberless connection.
When the two peers (computer and modem) are connected for this target, that cable cannot be used for transfering other data except the one coming from the internet
that's the problem.
all computers connected with the wlan should be able to talk to each other, while the computer actually involved in the establishmet of the connection is cut out.
This combo device is somehow unsual since most are dsl router and can establish the connection by themselves, without any help from a computer.
a) buy a combo dsl-router/wlan-access-point (i suggest linksys for quality/price)
b) use the computer that establishes the connection as an access point for the others (and this means some sostatial modifications in the logical topology you're using now)
Unluckly, I suggest solution A if you really need to use the wlan thing and this means spending some money...
Thinking carefully, this device seems to be too strange: try to see if it can be configurated to act as the pppoe origin or it is only a passive device