i need help switching to a new device

long story short, my phone broke. i have a backup device i can use until i fix my phone. i confirmed its compatible. its stuck with it under my devices as my replacement device. when i try and replace my current phone, it says the imei isnt active because its stuck under my devices. when i try and activate that device it tries to start an entirely new line. please help i am getting so upset. i need the iphone 12 removed from my devices so i can switch my service from the 14 to the 12.