The overnight chlorine test gives a good indication of the cause of chlorine demand. Sunlight always consumes chlorine during the day. Over night, chlorine is only consumed cleaning up the water.

The process:
Test for FC and CC in the evening after the sun is off the pool. Be sure that and chlorine you've added has had a chance to disperse throughout the pool.
Test for FC and CC again in the morning before the sun is on the pool.
The clean up is done when you lose less than 1 ppm FC overnight and there's 0.5 ppm or less CC.

When the chlorine holds overnight, I'd start to bring up the CYA to the target level. Adjust pH and CH as needed.

As I understand, SWCG's don't work when the water's cold. Do you have a owner's manual for it?

What are your FC, CC, TA, and pH readings now