Hi! I looked at the recipe you linked to and would definitely not try adding cranberries to that cinnamon sauce. You’d use the La Lechera product when you want something sweet, dairy-tasting, and perhaps caramel-like—think of flan. To my palate, cranberries are too tart to play nicely with that concentrated dairy flavor profile.
What kind of cranberries do you have?
If fresh, you might want to make a separate cranberry sauce with citrus in it as @Lambchop suggests. Serve that on the side so your guests can add an amount to their liking.
If you have dried cranberries (aka Craisins), consider chopping them up, maybe with walnuts, for a granola sort of topping on your finished French toast. My favorite breakfast place does something like that.
ETA: The sauce or the canned condensed milk (La Lechera) looks like it’s meant to replace maple syrup—agreeing with @Rooster here. Though if you have maple syrup handy too, you could always serve both sauce and syrup and let your guests choose their own adventure.