Find My disabled on new iPhone despite being enabled

Ive changed phones recently (gave my old one to my mum which is now connected to hers apple id) and now Find my is disabled on my new phone apparently. Ive received and email saying “Find my has been disabled on iphone. With Find My disabled, this device can no longer be located, placed in Lost Mode or remotely erased using icloud.com/find or the Find My app”.

All the settings for find my are on on this phone and the app shows my iphone is with me. However, my mum cannot add me to share their location with me as it says my number is not connected to find my app and I cannot add her either as she is not on the list. Any idea how I could fix this, please?

[Re-Titled by Moderator]

Original Title: Find my disabled