Un-ethical non-refund of excess amount charged - for last 1 year.

I took Xfinity Internet + Xfinity mobile in Sep, 2022 (with 1-2 days gap). Last month - I realized that I was not getting the promised 10$ discount on Xfinity Internet (10$ bundle discount as I have Xfinity mobile) since Sep 2022. Last month I was informed that I should get a refund of 120$ and which will be applied in my next month statement. However, I still didn't see it. Today when I called customer care, they are making excuses and instead asking me to purchase another line to get discount. I don't have any extra line. 

Is it NOT purely un-ethical to NOT provide the promised/advertised/agreed discounts ? I can understand there was a billing error and probably I also missed noticing it. But when highlighted - is it not right for me to demand the extra amount charged and get it back? Again from your perspective 120$ may not be a big amount, but it is more about what is RIGHT. 

Note: I have called customer care/billing multiple times - and after waiting for multiple hours - but this does NOT seem to be going anywhere. Hence raising it here and looking for suggestions on how I can deal with it.