2.4 ghz wifi question

I have the Panoramic WiFi, which supports both 2.4 ghz and 5 ghz network.  Anyway, my understanding is that if you are using a device (i.e.: Cell phone) that is only capable of accessing the 2.4 ghz band, then you are stuck at the slower speeds even though the Panoramic WiFi goes up to 5 ghz, which is faster.  Anyway, I recently chatted with a Cox representative and he said that Cox offers a plan where you pay $15.00 more per month to access the 5 ghz network.  Does that plan actually override my cellphone's 2.4 ghz limitation and allow for faster speeds over wi-fi, like the 5 ghz speeds?  I currently have the 150 mbps coverage, but I'm not sure if that is strictly through an Ethernet connection or if that includes Wi-Fi.  Is what the Cox representative trying to sell me for $15.00 more per month legitimate?  He was actually claiming that I could get speeds greater than 150 mbps over Wi-Fi with the "upgrade".

Thank You.