Using Outlook for iOS in business environments for email and scheduling
Had the same issue on my iOS device once my mailbox was migrated from O365 to on-prem Exchange 2016 (don't ask...). Specifically, I was getting "Unable to log on" in the iOS app, despite having all of the entries (email address, password, Exchange) set correctly.
For some reason, I needed to go into Advanced Setting and specify both the domain and my ActiveDirectory UserID. Tricky bit: even though the same text seems to indicate that the "domain" they're looking for in the Advanced Settings is the FQDN (i.e. sample.com) what they really want is your ActiveDirectory domain (i.e. "sanfrancisco"). Further, while the sample text in the username field indicates that they want the first part of your email address (i.e. first.last) what they really want is your ActiveDirectory userID (i.e. fname). Once I entered these bits, I was able to authenticate no problem.
Hope this is useful for someone in the future!