@Paul Suarez The problem has nothing to do with DNS. Synergy simply connects through the first working network connection that it finds. It's pure coincidence that it didn't use the manually configured ethernet connection. It doesn't matter if there is DNS configured or not. As long as one of your connections has valid DNS configured, the cloud discovery feature will work.
I think we'll be forced into doing this sadly, but it'll be a hidden/advanced setting for sure. Going forward, we'll probably try implementing a feature to test all available connections simultaneously, choosing the