Text message forwarding not working between iPhone and iPad

Hi, I have an iPad and an iPhone, and for the past few days I have only been getting message notifications on my iPad. When I go to Settings>Messages>Text Message Forwarding on my phone’s settings it only shows my iPad listed, but doesn’t show anywhere I can add my iPhone or disable this for my iPad. In my iPad’s settings, there is no Text Message Forwarding for some reason. Both devices have messages synced to iCloud

[Re-Titled by Moderator]

Original Title: Why am I only receiving message notifications from one device and how can I fix it?