How can I tell if I’m 2.4Ghz or 5Ghz

I replaced my Xfinity modem/ router one year ago ( Xfinity equipment ). I have a lightbulb that can be controlled with an app, and it needs to pair with 2.4Ghz. Since I replaced the modem/ routerI no longer see the option to choose 2.4Ghz. How can I see this ?