Unstable Wi-Fi with Core Fiber

I recently upgraded my Wi-Fi to Core Fiber with Telkom as I was told my old contract was out of date - the old connection worked perfectly for many years but since having a new router and modem installed my wireless connection to my household devices is very unstable with the connection dropping regularly. I have reported the problem numerous times but all their technical department wants to do is refresh the connection over the phone which does NOT help. When I asked for a technician to come to my house to resolve the problem, they want me to pay a R1,200 call out charge.