Your cable company sets the allowable speed by configuring the software inside your cable modem.
Do you have a docsis 3 cable modem? They are inexpensive used on ebay, much less than than your cable company will charge you. Once you get it, you have to register or provision it with the cable provider.
I had this problem when I upgraded to a higher speed. They were confused as to what speed I wanted because I did it twice. They even sent a tech out who said the line was ok, and who also told me that my cable modem was not "big enough". He thought it was physically too small, and they charged me $60 for him to come out. To eliminate my home wiring from consideration, I connected my cable modem to the cable where it enters my house. Then I connected it right to the cable companies equipment to eliminate the outside underground cable from their box to my house. I did this because they tend to first blame things that don't belong to them, or things that can make them money if they fix it. "Oh you have a bad cable modem, or router, or wifi, or house wiring, or computer."
Eventually they reconfigured the cable modem, and when my laptop is connected directly, or to my router directly I can get over 100 Mbps now. However, due to wifi limitations, I only get about 55 Mbps download speed via wifi on my 2009 Macbook pro. I have an 802.11N router with wifi.
Try putting your computer right next to your MacBook air and see if the speed is any faster from where you tested it before. Also, try someone else's laptop that has an ethernet port and use an ethernet cable and see if they are getting faster speeds. Or buy the dongle and try that. Or go somewhere you know the speed is faster because someone tested it, and try a speedtest there with your macbook air.
If you tested by connecting directly via ethernet cable into your router and the speed is what they claim it should be, then the problem is with the wifi portion of your router. If it's not faster, plug it directly into the cable modem. You will have to reboot the cable modem first after plugging directly into it. This way you are eliminating your router/wifi/switch from the chain.
You have to do this troubleshooting to narrow down the source of the problem. The people you talk to at the cable company want to sell you things that make them money, it's part of their job so you have to decide what you need, not them.