How untangle two Whatsapp accounts created with the same numer?

First, I installed Whatsapp on my Alcatel phone, with my whatsapp account ("son").

For my old mother, I bought her a big screen, cheap second hand Samsung Galaxy, (poor eyesight). That Samsung is blocked by the phone company (long history, don't ask); it can't make/receive calls.

I installed Whatsapp (via Wi-Fi) on her phone using my own phone number (stupid mistake) to get the audio check code. She got her own account ("mother"). It works almost fine, but I ("son") can't add new contacts because invite responses goes to "mother" account.

I changed phone number in "mother" whatsapp to her real working Nokia basic phone, but still doesn't work.

Short of deleting "son" and "mother" Whatsapp accounts, or buying a new SIM card for me, can you think another solution? I live 4 hours away from "mother". I don't want to lose 2 years of messages and contacts.