Modem cannot support the service I'm being billed for, and I never requested nor approve and upgrade.

I have been a loyal Supersonic subscriber since 2020, consistently paying a monthly subscription fee of R1,100 for a 100 Mbps upload and 100 Mbps download package. Recently, I received an SMS indicating that my account is in arrears. When and by whom was the subscription amount changed from R1,100 to the current charge? When did I approve a change in my subscription tier or service level? Why am I receiving arrears notifications despite uninterrupted monthly payments since the inception of my contract? Additionally, the router currently in my possession a Zyxel DSL router (model image attached for reference) has not been upgraded in over five years. Based on its specifications and current configuration it cannot support 1000 Mbps upload/download speeds. The router's Ethernet ports and internal hardware do not support gigabit throughput. As such, it is technically incapable of delivering the 1000 Mbps service level that I am allegedly being billed for. Given this, it appears I have been paying for a premium service tier that my current hardware cannot support, and for which I neither requested nor gave informed consent to upgrade. I requested a full review of my billing history,but nothing has been done and my Fibre service is inactive now for four days costing me lots of money.