Sorry this slipped through the cracks, @John_Zurowski. The instructions recommend using Let's Encrypt to obtain a free certificate. Is there a reason you can't do that?
It looks like something in Enketo prevents the use of self-signed certificates. @martijnr do you know whether that's one of Enketo's libraries, an nginx configuration, something else? Is it a deliberate design decision? ODK Collect currently does not recognize self-signed certificates either but this is under review (e.g. Odk Collect with Custom Certificate: Android 5 (works), Android 7,8 (fails)). I'm working on putting together a reasoned policy and it'd be great to have that be consistent between the two clients.