Did not receive promotional discount and mobile package increased

I called in to get a new promotion on our home internet. The agent made that happen. The she said if I added another line to our mobile service we'd get another $10 per month off for a year. i.e. save $120. I asked her multiple times whether it would affect the pricing on our mobile service and she said no it would not change. But it has gone up. And I've not received the promised $10 off promotion.

I spent two hours on a chat trying to resolve this and was passed back and forth between mobile and home internet agents multiple times. I asked for a supervisor who said they'd get back to me in 24 hours. I never heard back.

Then I went to the local store in person. They said they couldn't help and to try the phone agent.

I called and was put on hold for two hours while I waited for a supervisor. She offered me less than half of what I was promised as a credit. I declined. The supervisor put me on hold again for another two hours and never came back on line to talk to me again.

Nearly all of these agents and supervisors do not have the authority to resolve these issues. Also it seems that the mobile and the home internet people can make promises about the other service that they can't fulfill.

It seems that Comcast/Xfinity either trains their agents poorly so that they sell promotions to customers that aren't eligible for them OR they intentionally train their agents to lie and commit fraud. 

Either way they then make the customer liable for the agents incompetent or fraudulent actions. It should not be up to the customer to root out whether an agent of the company is offering them a promotion that is valid or not.

It is terrible customer service. 

I'd like to know if there's a way to get the $120 credit I was promised and return my mobile billing to what it was before I was sold an extra line that I didn't need.

The original agent and I communicated via chat, and I copied the chat so I have a copy of what she told me on record.