Sharing between a speedstream DSL modem (USB) and a NIC is exacly what I'm currently doing (next house, we'll be moving to a nic-based modem so I can use linux...speedstream drivers are windows only).
here is how I have it set, the common mistake is to select the internet shared device as the speedstream modem itself, but you need to set it as the PPPOE driver (attached picture)
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