Remove new id and get the old one
Remove the new apple id
iPhone 6s, iOS 15
Use the article below as a guide of where to sign out of the new ID and then sign in with the old ID. Make sure you turn off Find My Phone before signing out of the new ID. Once signed out and Find My Phone is off, then sign in with the old ID, then turn Find My Phone back on.
What to do after you change your Apple ID or password - Apple Support. *