Modem not Transmitting Gigabit Speed

I am currently getting Xfinity Gigabit Internet using a SURFboard SB8200 and 3 Mesh Google Wifi routers (one connected to the modem, the other two on each floor on top of each other.) I had an Xfinity tech come out today to check my service since I've been getting 300 Mbps (worse on the second and third floor. The tech confirmed that my router is receiving gigabit speeds, but isn't sending it out to my routers. I tried looking to see if I needed to upgrade any firmware, but I could not find that or anything of support on Arris' website. Does anyone have any troubleshooting ideas as to why my modem is not transmitting the speed it is receiving?