We have a test portal setup that uses internal SSL certificates. I have loaded the certificate on my iPhone and turned on the Trust Settings. When I attempt to login using the iPhone and Mobile App i get the following:

Is there an issue using Internal Certificates and Esri apps? I am having the same problem with Workforce as well.
Or is it beacuse we are using the fully qualified domain name for the portal rather than https://webadaptorhost.domain.com/webadaptorname
Thanks,
--gary