Hello again everybody - I managed to get everything working, based on this guide in another thread. I also found that the latest desktop version of 2.1.7 only plays nice with the latest beta of joplin server (2.1.6-beta) according to this post. You can change which version of joplin server docker pulls down by changing the "latest" tag in the docker-compose.yml file to "2.1.6-beta". Current versions of apps/server playing happy together below:
Joplin Server: 2.1.6-beta
Android: 2.1.3
Windows Desktop: 2.1.7
iOS (ipad OS on iPad Pro 2020): 12.0.2
Ubuntu 2020.04: 2.1.7
Thanks all, and let me know if I can help.