Not getting speeds payed for

Ive been an xfinity customer for about a month now and my download speeds have been dropped 15-100mbps, after a reset on the router they stay at 60-100 for a while but then drop down to an average of 30mbps, ive looked everywhere about this, and im not the only with this problem, the speeds used yo only stay at 30mbps so about a week ago i started an online chat with an xfinity employee who troubleshooted me a couple options and ended up reassigning the router or whatever and it bumped the speeds up to 100mbps he then told me to reset the router which i did and then he tried a few other things and when he didnt have anything else that could help he simply ended the chat! which has brought me to now where i get speeds 15-60mbps and 30mbps on average, its not my modem/router because ive seen speeds go slightly over 300mbps in the beggining and my moden supports 300 mbps, upon checking it multiple times today its staying at 10mbps after a modem reset and in general, please let me know if you can help and what you need, thanks in advance