The issue is that users trying to connect to a SharePoint 2016 site using https will be continually prompting for credentials when browsing using an Apple iOS devices (iPhone, iPad, etc).
We have spoken to Microsoft about this issue and appears to be a known problem and came back to us with the following response.
This issue occurs as the SharePoint Authentication is not compatible with the IOS
As of now this issue has been reported but the change is required from the Apple end, such that the IOS accepts the SharePoint Authentication.
They recommended a few work arounds, but it appears that there are a lot of cases with similar problems, so I was just wondering what work around others are using, until a fix is put in place.
Using Http url for SharePoint
Using a Web Application Proxy for SharePoint https://blogs.msdn.microsoft.com/sambetts/2015/07/01/sharepoint-server-and-web-application-proxy-continued/
The current work around for me is to use a web proxy but this pain as it needs to be updated on each device for each connection.
I had exactly the same issue with HTTPS and SharePoint 2016.
The issue was solved by disabling HTTP/2 for IIS 10. I had to create two registry keys: under HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\HTTP\Parameters
EnableHttp2Tls REG_DWORD 0
EnableHttp2Cleartext REG_DWORD 0
Update
This should only take effect after you restart the server.