Okay temps are FAR too cold. Aim for 80-84F during the day and no lower than 75F at night.
As African bull frogs get a lot of D3 in their diet UVB might not be necessary but considering they're active day and night, exposed to the African sun, UVB wouldn't hurt but they have been kept without it and been fine when fed D3 in their diet but I wouldn't consider this a study but more an experiment with supplements, so I don't think there's actually been any studies on their UVB needs.

Stick on heater, a UHT? A UHT/heat mat would usually get temps up to 100F when used without a thermostat but will vary with brands, though surely higher than 68F. Remember they're contact heaters and will get the glass hot but not do much to the air temperature, so you must make sure you use a digital thermometer (not analogue) and put the probe directly on the glass being heated by it.

Changing the substrate once a month would be better, big frogs produce a lot of waste.