SB6141 modem speeds slow than expected (May 2022)

My customer owned SB6141 is no longer on the supported equipment list but it seems to me my bandwidth was much higher back in the day now I only get 120 Mbps speed tests on a desktop computer connected via Cat5 Ethernet cable. I have 300 Mbps service and Xfinity says my modem is capable of 284 Mbps. Modem signal page shows 35-38 for all downstream and upstream channels. I'm obviously not expecting gigabit bandwidth but I think I should be able to get better speeds with my existing equipment. Any suggestions would be very much appreciated.