Product: Credit card or prepaid card

I applied for and was approved for an American Express card - specifically, a co-branded Hilton Honors card. The promotional terms advertised by American Express in the link that I used to apply were stated as follows : " Special 0 % Intro APR Offer on purchases for 12 months, then a variable APR of 15.74 % -24.74 %. '' The interest rate promotion has never been applied to my account. I have been contacting American Express since XX/XX/XXXX and no one from the company will provide an answer or follow-up with my customer service inquiries. 

I have provided them with screenshots of the offer I used, the original e-mail sent to me with the promotion, and a URL link to a live webpage which shows the offer that they have repeatedly claimed that they don't know about. 

Their customer service representatives claim that a " back office '' handling these types of issues has to research it and that they will follow up with me - and then no one ever does. I have already made a full statement payment because they won't honor their advertised promotional rate or follow up on my inquiry. 

Timeline of my attempt to resolve this issue with them : XX/XX/XXXX - Card Approved XX/XX/XXXX - 1st Attempt contacting customer service confirming offer - they say they will follow-up. 
XX/XX/XXXX - AMEX representative XXXX XXXX XXXX asks me to e-mail proof of the offer. I comply and do so the same day. 
XX/XX/XXXX - I call to follow-up since I never received a response. Customer Service Representative tells me " no one will ever follow up with me - I just have to keep calling ''. 
XX/XX/XXXX - I follow up with XXXX XXXX XXXX via e-mail. He never answers. 
XX/XX/XXXX - I call again. AMEX acts like they have no idea what I'm talking about. Promise follow-up, never happens. 
XX/XX/XXXX - I call again. Still no answer, promise follow-up. Never happens. 
XX/XX/XXXX - I call again. Representative XXXX XXXX asks me to e-mail the proof again. I do so, never heard from him again. 
XX/XX/XXXX - I call again. Representative XXXX XXXX XXXX asks me to e-mail the proof - for the third time. I do so. He says he will follow-up by Tuesday, XX/XX/XXXX. He of course does not. 
XX/XX/XXXX - I e-mail asking for follow-up, again. XXXX gives a reply about needing to escalate it further and how he was too busy to respond as he said he would on Tuesday. But he does at least respond to my e-mail at this point. 

It has been over 7 weeks and every time I call I get a different, evasive answer and they never follow-up. I have been their customer for 8+ years and the offer is clearly advertised on their own website - and is currently being advertised at XXXX. There is no reason why they should be confused or unable to follow up with a simple customer service request. They assured me when I signed up for the card that the promo rate would be applied and I shouldn't worry about it. I would have preferred to resolve this with them and have tried extensively. 

I would not have opened the card account if not for the promo rate. I feel like the company is advertising a promotion which they refuse to honor, then refuse to follow-up with customers until they give up or stop calling.