How do I disconnect my iMessage from another phone

I signed into my Apple ID on my grandmother’s Iphone to download an app for her because she does not have her own Apple ID. However now my iMessages are connected to her phone and she is receiving my iMessage that I receive on my phone number. I checked the settings> Apple ID> name, phone numbers, email

and her number appears under “reachable at” when I try to delete it, the red delete bar does not come up so I deregistered the phone number by sending a code— but this did not stop her from receiving them— how do I disconnect my iMessage and FaceTime from her phone number?