Upgraded my package, but I'm still not getting advertised speeds

[cadestrum]

Making this post to see if anyone either from community or from Comcast themselves to help; I've called about 5-6 times to get to Tech Support about the issue but no one has been able to actually help. I upgraded 2-3 days ago and have since then restarted my modem multiple times, but no success in speed upgrades. The downspeed upgrade was from 300 Mpbs to 600 Mpbs but that's not the area I'm concerned about, I upgraded upstream speeds from 5 Mbps to 20 Mbps ( I was promised 35 Mbps but whatever.) I still can't get past 5.5-5.9 Mbps.

I'm suspecting it might be the configuration file is not being pushed to the modem, but honestly I can't say for sure.

Using a Cat6 Ethernet cable

I have tried multiple computers

I have a Arris SB6183 which should support the speeds I'm trying to achieve.

I have a switch connected to the modem, but I know it's not the switch because I connected directly to the modem from PC to modem, and still getting same results.

[CCAndrew]

Per, this link, your modem isn’t compatible with the new plan
https://www.xfinity.com/support/devices/#unauth