Never mind. While I am still curious as to what is needed to get it to work, I gave up and created an account at cloudclusters.io and it worked on the first try.
Given the other providers are all running 3.x of MongoDb and they are running 4.0, I kind of wonder if MongoDb Inc. is running some kind of "purposely-incompatable-but-still-open-source" game to stop the other cloud providers. Just a guess given some of their other behaviors recently,
