Poor service between Vumatel and VOX - client needs to resolve

Vumatel upgraded their fiber line in my area, Emmarentia at the beginning of the month (May-24). I notified my ISP (VOX) following the upgrade that my line speed decreased from 100mbps to 50bps. Both Vumatel and VOX refused to take accountability of resolving the problem. Vumatel have a monopoly in the area and believe poor service delivery is acceptable, whilst VOX my ISP are comfortable passing the buck.