This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 曟 is uncertain. It visually combines 晶 (three suns) with 辰, but the original semantic and phonetic roles are unclear.
With three suns 晶 over 辰, imagine a bright morning scene, but the exact meaning is unclear. Think of it as a mysterious kanji related to morning or time.
For シン, picture the three suns shining (シン) brightly in the morning sky: シン -> シン, a simple sound hook for the reading.