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 ...
Plan. Every network-capable device has a network adapter (wired or wireless). This adapter is a piece of hardware. Cox has no plan to "override" a piece of hardware.
Speeds. You must divide your Internet service into 2 networks. One is the Cox network and it extends from your wall-outlet and connects to your modem. Basically, you have no administrative control of this network. Cox sets your service at 150 Mbps and Cox can "see" your modem and if Cox explores, Cox can detect a piece of hardware is actually connected to your modem.
The other network is your personal, private network. Basically, this is your router. Although you're renting this router from Cox, Cox has no administrative control of your network. Meaning, Cox has no control, ability or obligation for the speed of whatever is wired to or wirelessly connected to your router. Cox is only promising 150 Mbps to your modem. The bandwidth of your network is your responsibility.
I don't know about this $15 plan. I thought it could be Cox Complete Care but this is only $10...and not what you want. I'll assume the $15 is to upgrade your 150 Mbps to 1 Gbps. However, again, 1 Gbps would only be to your modem and not to your cell phone.
Bottom Line. Your phone can only use the 2.4 GHz band. Upgrading to 1 Gbps could boost your private bandwidth but there are better ways to boost than shelling out more money to Cox.