Vox Telecom worse service

I have fibre contract with Vox since from February this year. I have paid connection fee when they connected my service for the first time. Last month I decided to downgrade my line from 20/2 to 10/10. The downgrade became effective from June 2019. My bill this doubled from what I have been paying due to pro-rata amount that they can't even explain and I had two days without service as they were busy with the downgrade. Pro-rata amount for me should be less amount as I downgraded not upgraded. Please assist with this matter as no one is willing to help me.