Huge billing issue

As per previous review which didnt help me at all. Ref *** and mtn complaint ref *** - mtn advised last week that there is nothing more that they can do for me and that i must pay the outstanding amount which is really not my problem nor mistake as they cannot get the RMA no due to ZERO feedback from mtns side. They advised last year oct when the billing query was logged that this WILL be sorted out and now 4 months later , i must pay the amount? Because they cannot get feedback? Someone is probably using that router and now im getting billed for it

I need this to be sorted out ASAP