Slower speeds after upgrading modem

I am getting slower speeds after upgrading to an Arris SB8200 modem coming from an Arris SB6183.  I was getting over 400Mbps down and 20Mbps up with the SB6183.  Now I am getting 350Mbps down and 12Mbps up with the SB8200. I am on the up to 900Mbps plan