Being charged for fibre speed that I am not receiving

I have a 1000mbps fibre line with Afrihost, which I upgraded in May. To date I have not been getting the spped that I am paying for, if I get 300mbps I am lucky. I lodged a complaint with them about a month ago, they then requested Vumatel to come and check my fibre line, of which they were vey efficient. Vumatel was at my house the day that the issues was reported to them by Afrihost. They then told me that there was nothing wrong with their line, the issue was more with my ISP. The issue was the escalated to the ops fibre team and to my disgust they referred the resolution to my query to the visit that was done by Vumatel. Its been a month and to date nothing has been resolved. They then expect me to pay my invoice in full when I am getting a raw deal from them.