Mtn bad service provider

Mtn is one of the worst services provides regarding customer care. My banking details changed from Absa to Fnb. With my uncapped wifi i need to change the bank details as the amount is paid via debit order. My fingerprints do not work on the system, apparently they cannot assist me in the MTN shop (Heidelberg Mall). They say i must call 135 for assistance. My cell phone is with Vodacom , does not help me to phone 135 as this will call vodacom service. What can be done to change something simple as this.