Cell C's Frequencies

I notice that my Cell C SIM doesn't work in slot 2 of my HTC One dual SIM phone. So what frequencies do they use. Slot uses these,
GSM 850 / 900 / 1800 / 1900 - SIM 1 & SIM 2
GSM 900 / 1800 / 1900 - SIM 1 & SIM 2

On the phone.