It may be a similar issue to this post [BUG Confirmed] LocalNotifications don't work on iOS 10 in 4.17.1 - Mobile - Unreal Engine Forums
I haven’t gone into the source and checked the APIs for remote notifications, but the local notification ones that unreal uses were deprecated in iOS 10. Given you have it working in 9, I’d hazard a guess that the APIs used by unreal for remote notifications on iOS have been deprecated, too.
Cheers,
Alan.