Badly positioned internet installation, refusal to fix despite numerous calls

On 18 Sept Vumatel installed a fibre optic cabling and a box at my home so that I could have a fast and efficient internet service. I had pointed out, even prior to installation, that the box needed to be positioned so that the wireless internet service would cover both my home and a cottage on the property. I duly paid their invoice for installation in full on presentation. On 3 October my chosen internet service provider, Vox, connected me to their service via the Vumatel-installed box. The wireless internet service is good and fast, but it immediately became apparent that it did reach my cottage. On 6 October I emailed both Vumatel and Vox to say that the coverage was inadequate, and that the problem needed to be addressed. Vox responded promptly by sending a technician to check that their service was adequate, which it was. However, to date I can get no resolution of the problem from Vumatel, despite 4 phone calls and a series of emails asking when I could take a phone call to discuss the issue. Initially I was told that I would have to pay an additional R4 000 to get the internet coverage to my cottage, with no admission that their installation may have been incorrectly positioned. However, beyond their insistence that I cover all the additional costs, I have had no communication as to when, and indeed whether, the problem will be fixed. The box needs to be moved at most 5 metres. I am way beyond exasperated.