Vivint APX Alarm Failed to tell me how to cancel until deadline past for auto renewal.  Internet

I originally signed a 5 year contract with APX Alarm.  I did research them beforehand and I did see a few complaints but not very many.  The installers were polite and the system seemed to work fine.  A couple months before my contract was up, I asked about canceling.  They said I needed to contact them closer to the date that my contract will end.  I contacted them a week before it ended and they said I needed to send an "Intent to Cancel" letter which they did tell me how to send that in.  They never had the courtesy to remind me that any letters received after the date of auto renewal would be void and completely ignored.  I sent in the form and a couple months later, they are still charging my credit card.  When I complained they explained that my letter of intent to cancel was received 4 days after my contract automatically renewed for another year and there was nothing that they could do.  They kept repeating that there was a South Carolina law (I'm in SC) that they had to follow and they could not change my contract in any way and they kept repeating that the information about auto renewal was clearly on my original contract that I signed 5 years ago.