Arris SB6183 modem speeds capped

I use ARRIS® Surfboard® SB6183 Cable Modem, White. Spec: Get speeds up to 686 Mbps download and 131 Mbps upload (speeds based on your Cable Internet service provider). Compatible with most U.S. cable providers.

I am only getting 90Mbps download 25 Mbps upload. My plan says 300MBPs.