SLOW fibre speed

Terrible customer service, I received the incorrect modem from MWEB, now I'm paying for 100 mbps, and only receiving intermittent service of highest 67 and LOWEST of 4 mbps. They should send me the correct modem, but instead they keep blaming the installation (tested, works perfectly). Even their technician said it's the modem..... And trying to phone in to their customer service / sales department;(((((