Defaulting to 2.4, not 5GHz

[shuuby]

Got a new modem Saturday 5/16, and unlike the dual network options I had before where I could choose to use 2.4Ghz or 5.0, this new set-up only seems to have one network. 

But it keeps defaulting to 2.4. I got on chat Saturday and a tech did some tech thing and I re-logged and had 5.0 (My d/l speed improved from 40m to 220). By Monday I was back to 2.4 and the chat person today may have been a useless buffoon.  He ordered a new modem, but I have little faith that that is the answer. 

Any settings I need to know about? Thanks for any help!  

[EG]

If you are not using any Xfi Pods, you can try separating the 2.4 and the 5.0 bands and give them two different broadcast names / SSID's;

https://www.xfinity.com/support/articles/change-wifi-mode-admin-tool-xfinity-xfi

[shuuby]

Thanks. I tried and wasn't sure what my network names were now, but I tried signing on to a couple things to no avail.