I'm not getting the speeds im paying for?

I recently upgraded my modem to a docsis 3 modem about a month or two ago. Well the guy on the phone said when you upgrade to docsis 3, your internet speeds get doubled for free.

I just ran four speed tests. I did two using the cox speed test, and I did 2 using ookla speedtest. I did 2 tests using a wireless connection and 2 tests on a wired one. I have the preferred plan which says it should be 50mbps download, and if that got doubled then it should be 100 right? or was it at 25 and now its 50? Well either way, I'm not getting either of those speeds even though I'm paying for that plan. The highest download speed I got was 28mbps using a wired connection. The highest wireless was 23mbps. My modem is fairly new and so are my Ethernet cords. Nothing is broken on my end and there's no disrupting variables. The modem and router is 4 inches away from my pc. How come Im not getting higher speeds? Linksys E1200 router and a sb6141 modem.