Just a data point: AirPort Utility 1.3.5 (135.24) under iOS 3.1.2 connects without issue to my fourth-generation Time Capsule and my various-vintage collections of AirPort Express stations.
I have had instances in the past when I couldn’t connect to the Time Capsule with AirPort Utility on iOS or macOS when the Time Capsule otherwise appeared to be operating normally. Power-cycling the Time Capsule was required to get connectivity again.