I know others have asked this question, but I don't know that my situation is the same, or at least the suggested answers didn't work for me.
I have my hmdm set up with my domain, mdm.mydomain.com which is behind nginx proxy manager for managing the SSL cert.
I can visit my site, log in, get a QR code, do any of the things I would expect to be able to do.
When I scan the QR code, I get the error that the admin app can't be downloaded.
I am using HTTPS, my links are using HTTPS, I can download the links using the URLs in the application list to my server, and from any other internet client from my domains application URL.
I've tried 2 different devices (Samsung Galaxy Tab S2 and Samsung Galaxy Tab A) on 2 different wireless access points (my home's wifi and my cell phones mobile hotspot) and I get the same thing on each device and each connection.
I've tried changing the application URL from HTTPS to HTTP - didn't work
I've tried changing the baseURL in the tomcat ROOT.xml - didn't work
When I scan the QR code with my phone, I get the appropriate (or at least I think I do) link to download as it is referring back to my website.