MTN / Afrihost confusion

Confusion reigns!! I have an Afrihost fibre line for use at work/home. All my devices are linked to the wifi. When I have load shedding my devices 4g should take over but they have a very poor signal. When I buy MTN data and set their API I can use my 4g. Is there an explanation for this as both Afrihost and MTN use the same towers? Regards Detlef