Dual band wifi 2.4 and 5.0GHz

I  picket  up today a modem at the Xfinity branch that is supposed to have dual wavelength. so that I can install a Simply Safe security system that only works with 2.4 GHz.  I was told that following the installation instructions  I would be able to configure the modem for dual  wavelength.   However; after finishing the installation  there  were no instructions on how to set up the dual waveband so that devices using 2.4 and 5.0 GHz can work simultaneously