Samsung sm G906k ussd code not working

Hi they offered me this samsung s5 prime LTE-A korean version from allo network. Everything is working fine except for the ussd codes. I am in cameroon using mtn network provider. When i type a ussd code to check my bundle or airtime balance it works but when i type a ussd code to activate a bundle it says try again. I flashed the phone with the stock rom but still have thesame outcome. So i am out of solutions. Please can someone help me

Envoyé de mon SM-G906K en utilisant Tapatalk