Users of IOS can't log into a SSRS2017 Report Server using either the Power BI App or Safari browser. However, If the same account is used to log into a window browser or Android Power BI APP then it works fine.
Window Server 2016 Standard
Authentication mode in Rsconfig file: NTLM, Basic.
User accounts under Azure AD
What version of the SSRS do you have installed (you can get it from Help About in portal) also what version of the iOS app do you have? Are you getting an error when you try to connect? I just tried it with the following versions and it worked for me:
SSRS - 14.0.600.460
iOS App - 14.2
We have installed the same versions as you do have. What we have found is that the issue only occures when we connect to the portal on HTTPS, If we do it on HTTP it works, and again, only for Apple devices.
I hope this helps.