Hi,
We had a similar problem recently and found it was an issue with the version of Webkit on iOS.
Our prototype which had been working in the XD Viewer app just stopped working on some devices. It affected the client and a number of other phones, but not all of them.
We installed the Adobe XD app which worked correctly, but this wasn't helpful since, for some reason, it's not really possible to use it to demo a prototype to a remote client!
We also installed a range of other browsers on iOS devices and had mixed, although unsuccessful, results. Finally, by creating a simple iOS app to display the public URL we were able to trace the error messages which identified the problem to Webkit.
We found that updating to the latest version of iOS solved the problem when viewing in browsers, but not for the XD Viewer app which must be using an older Webkit.
I suggest trying the prototype url in a browser on the phone and if that works then the issue is not with the prototype.
The end result for us was that the client had to view the prototype in a browser which is not ideal but worked well enough.
Hope that helps!