Unbearable customer service

My bill went up really high. I called retentions. They offered me a $20 promotion which sounded nice. Unfortunately it was not. They were unable to apply it because they were getting an error message. They told me to call tech support. So I sat on hold for an hour. Never picked up. So I called retentions back. They kept me on the phone for about 45min while they worked with tech support. Tech support's resolution? Swap out the modem. Great, replacement modem to get a $20 promotion. Then I got the modem they sent, no coax output. The service was installed with coax. I call the retention department again, unable to help. They transfer me to tech support. Tech support tries to find another modem, no option. So I told them to cancel the order and now I have to send the modem back. 3 hours on the phone. I wonder if my existing service will stay the same with the old modem since I don't want to give it up. I call back retentions, they tell me I just have to send back the new modem. They also were able to apply a $15 promotion. So a few people tell me "We can't apply promotions until you swap out the modem" and then another person applies a $15 promotion without issue. 

How do I know if I send this new modem that I cannot use back that they won't either charge me for it or disconnect my existing service?

How do I know if what these people are telling me (The $15 promotion) is even true?

Why is nobody on the same page. I cannot understand it. There is no reason to be run around like this over a small promotion that they offered. I'm still confused. I'm still frustrated.