Put some ebooks on your phone and read those. Stop mentally associating the phone with the infinite novelty generated by algorithmic social and YouTube feeds.
Do not open your web browser unless you have a specific search query in mind.
Disable all notifications except for the calendar and clock.
Treat your phone as if it had no online connectivity. When you open the phone, use it to organize your notes and structure your thinking, or to look at photo gallery. Begin thinking of it as an extension of your brain instead of a bottomless anti-boredom device.