telkom demanding money for the service they did not render to the client and bad signal in their network

I took a sim card only contract with telkom, to use for my works during covid in the 2021. I could not use it since day one because of no signal. I called and asked to helped and I was told my area has no signal.

I struggled till I had to get another ISP called vnc because I had to work from home but with telkom, I could not get signal and I nearly lost my job because of that.

That was the last time I used the sim card till now, but to my surprise telkom is demanding money for the service not rendered. How? Why? Please help me stop telkom from harassing me. I don't owe them.