Defaulting to 2.4, not 5GHz

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!