How do you value a company with a very high debt level?

For example companies such as DPZ and QSR have very high debt levels. If I do a DCF with normal or bullish estimates, it still arrives at a valuation much lower than the current market. In my valuation, after running the normal DCF, I added cash and subtracted debt. It was because of the subtraction of debt that led to a very low valuation since both companies had very high debt levels.

Am I doing this correctly to value these companies? Or should I not be adding cash and subtracting debt? If I am not supposed to, why not? Otherwise, is there any explanation as to why a normal valuation would yield such a low figure for DPZ and QSR?

My inputs and assumptions are not conservative at all and I just want to know why my valuation is so different than the market's and if I am valuing companies with high debt levels incorrectly.