I thought tab index functionality is a basic feature which should be working on all the devices for all the control types, but it is not working on ios devices(clicking up/down on the keyboard) for non-text like radio/dropdown controls, based on Powerapps Product group response, it is not yet implemented, so submitting this as an Idea.
Product support team response for my ticket.
"I checked with the Product group team and update given is the iOS implementation of these form navigation arrows does not use the tab system. Support for these arrows is not implemented in PowerApps.IOS restricts the types of controls that can be navigated using tab. In Mac OS, users can customize tab behavior, but unfortunately this is not possible in iOS."