IMA Pre-roll on iOS is supported, however in case the adTag has the "skippable ad" button enabled, it is not supported on the iPhone.
This is the error message that IMA test player throws when trying to play and skippable adTag: adsManagerLoaded Error: AdRules are not supported on iPhone ot iPad Ad Error: The provided ad type: skippablevideo is not supported. It appears that there is no way to make the skippable adTag to work on both platforms (e.g: Desktop and Safari iOS).
The feature "Skippable (non-TrueView)" is not supported on iPhone (as shown in URL below). https://developers.google.com/interactive-media-ads/compatibility Talk with your DFP representative in order to get guidance on how to disable this functionality, or ask if it is possible to have a tag that works on both Desktop and Safari for iOS.