Due to the changes made by Apple with the App Transport Security (ATS) requirements, some iOS 9 (and above) SSL application communications will require certain parameters for a server.  Here are the requirements NotifyMDM server to comply with the changes:

1. A SSL certificate using at least a SHA256 fingerprint (Advanced Encryption Standard) with either a 2048 bit or greater RSA key.

2. A minimum TLS protocol 1.2 version setup/enabled. Please note: SSL protocol 3.0, SSL protocol 2.0 version, and TLS protocol 1.0 may have to be disabled on the server hosting the NotifyMDM Web/HTTP component.

The change of the SSL/TLS protocol would require a registry change on that server.  Here is a link to a Microsoft Windows IT Pro Center article Transport Layer Security (TLS) registry settings ( https://docs.microsoft.com/en-us/windows-server/security/tls/tls-registry-settings ).  Please note, Windows Server 2008 and below/lower (Windows Server 2003, Windows Server 2000, etc) operating systems do not support the TLS protocol 1.2 version. Only Windows Server 2008 R2 and above/higher server operating systems support that version.

Once the SSL/TLS protocol change have been made, please reset Internet Information Services (IIS).