Any Jellies experienced with making jelly? What did I do wrong, and can I fix it?

Yesterday I attempted to make apple peel jelly from leftover peels and cores, following this simple method. Instead of jelly, however, I got, well, liquid. It never set up.

So, I picked up a box of pectin at the store, stirred it into my “jelly,” brought it to a boil for a couple of minutes, then put it back into the fridge. Now, I have syrup. It’s been more than 24 hours and it hasn’t set up. I only used half the box of pectin because I was making half as much jelly as the recipes in the box called for.

Am I stuck with syrup, or can I boil the mixture again with the rest of the pectin? Would adding more sugar help? I had about 4.5 cups of liquid and added roughly 1¾ cups sugar. It already tastes plenty sweet, but will adding more sugar help it turn into jelly? If there’s no way to turn my mess into delicious apple jelly hell, I’d settle for edible apple jelly at this point what can I do with my apple “syrup?”

If it matters, I don’t plan on actually canning the jelly, just storing it in the fridge until it’s used up.