MYMTN App errors

Please someone is using my line to share data through MYMTN app. I called the customer care and they told me to log into my device(I have not used the app before) and it will be logged out in the fraudster's phone.

I did as I was told and the person could still buy data with my number. I tried to login with another phone while still logged in the my own phone and found out that my account didn't automatically logout in my own phone as I was told it would be. Right now I am logged in three devices : the fraudster's, mine and the 2nd phone I used to test the customer care advice.

PLEASE HELP ME QUICKLY ANOTHER LINE IS LINKED TO MY LINE I THE MYMTN APP.

Please unlink the linked number. I tried unlinking via the app and it is not showing.