I did not get my new line 500 off iPhone promotion

I made the mistake of going into the store instead of setting my mobile account up on line. The guy in the store didn’t pay attention when I said I wanted an iPhone 15 pro and set my account up as a bring your own device. Now I will have to pay the full price of the iPhone instead of getting the 500 off that is the current promotion for new lines. is there anything that can be done. The whole reason I set up this new line mobile service was to upgrade my phone. I have xfinity internet for 5 years  and have had no issues but the phone service hasn’t been the best because of all this. The person at the store told me I would have go to a different carrier and creat an account to port my number and then come back and add a new time with the phone and re port that same number back to me in order to get the promotion. That didn’t seem right to me. Is there anything that can be done to keep my number I have had for 10 years and get the promotion I came to sign up for. I have only had the mobile service for 3 days and I’ve already spent 5 hrs in the store three different days. Thank you.