The first $18 modem by EAS worked:
https://www.amazon.com/gp/product/B012I ... UTF8&psc=1At least on this cheap one, the modem won't get its second light unless the Pi has an internet connection, so you can't test if just your DC->Pi stuff works. It has to be the whole system working.
The second $18.50 modem will be here APr 5-10, and the USR5637 will be here today, but we know that one works.
I'm able to use a WiFi dongle and connect to the internet on my DC, which I used to run "sudo apt-get update" "sudo apt-get upgrade" and "sudo apt-get dist-update" (the last two appear to do the same thing), but I RPZW still won't detect its on-board WiFI/BT chip.