Is there any flow/policy to prompt a user again to ask for local network permissions if denied earlier?

There is no way to request a new prompt from the app in the iOS 14 beta. The user can change their preferences in the Settings app. (Prompts can also occur again if the user resets their privacy settings).

This underscores the importance of prompting at the right time: prompt for access first in a flow when the user can understand why your app uses the local network, and can make the right decision.