Achieving 600Mbps download speed with Arris SB6190 model

Hi, I purchased the 600Mbps plan and per Xfinity recommendation on compatible equipments, bought my own SB6190 cable modem.  When I tested my speed it's at 230Mbps only with my computer connected directly to the wired modem output. I called Arris ane they told me that my upstream channels need to increase from 4 to 8 and power levels needs to be increased. Xfinity tech support tells me that SB6190 does not support 600Mbps and need Docsis 3.1 model even though thier own wesbite saysSB6190 which is Docsis 3.0  works for 600. And it seems they don't have a way to increase power unless I buy thier modem. Seems really disingenious to me. Any ideas or suggestions? Am I the only one with this issue?