I also encountered this problem in an on-premise environment with BC17, in which we just added the GlobalEndpoint setting in the web instance without solving the problem.
The webpage loads up to the login prompt but, when entering the login, the same credentials we use on Android/Windows devices (credentials are correct) do not work on iOS devices.
I may add another information about this issue: looking at the Security log on the BC server we encounter this message when attempting to login through an iOS device:
Tentativo del computer di convalidare le credenziali di un account.
Authentication package: MICROSOFT_AUTHENTICATION_PACKAGE_V1_0
Logon account: *****************
Workstation: WORKSTATION
Error code: 0xC0000064
This kind of error refers to "The username you typed does not exist. Bad username." but it's not, because on other devices the login works