I have one of these — disused and in a box somewhere or other — and once upon a time I did have it working. There is some small magic at play in the dongle itself that boots networking speed well beyond the ordinary limits of the localtalk/ modem port to which it connects. I don't think the increase was massive, but when the thing was new, it was enough to make the purchase seem worthwhile.

I do not think the EtherWave is compatible any form of Open Transport, and seem to recall that there are compatibility issues even with 7.5 (though that might just be the later iterations, i.e., 7.5.x). Maybe try under 7.1? I think I must have had it working on a Duo 280c running 7.1.

Some years ago, I posted on here and asked about the thing. The question got a very knowledgeable response (Gorgonops? Equill?). But that would probably have been before the crash, and the archive is not searchable.