Double billing and poor customer service

I have an LTE subscription with Web Africa, Last year November I pre ordered and paid for the the Fibre: Vuma Reach Uncapped 20/10 Mbps. The technician came today for the installation of the fibre. I went and cancelled the Fixed LTE: MTN 100GB + 100GB on their portal, to my surprise apparently the one month notice applies, this means I will be billed for both the services. One of their agents/consultants told me that there is nothing they can do to prevent this because fibre is already activated. The fibre activation is done by them not me, without giving me an option to switch services in order to cancel the LTE once the fibre is active. So in a nut shell I need to accept that I will be double billed and there is absolutely nothing they can do, which doesn't make sense to me at all.