Have extremely slow internet compared to what we pay for and cannot get any help from comcast

Our home internet is extremely slow.  Pay for gigabit internet and usually only get around 50 mbps for a download speed.  The modem/router has been exchanged for a new one and all of the cables indoors are good and not damaged in anyway.  It's also extremely difficult to get in touch with any live person at comcast.  I've even tried out another router and still the same slow connection speed.  We should at least be getting over 100mbps at a minimum.  Need some help with this.