The cable is only between the phone and the tv
It is still cellular data over the cell net work, to get to your phone
If you drive your car in the city streets and not on the interstate
It still burns gasoline
There are two issues at work here
And that is how your data is allotted by your cell plan
Amount allotted via phone display use
And limited via hotspot use
Everything you do on the phone uses data
Every text, every picture, every weather report, every app, every email
With today's 4glte . Phones even voice phone calls use data, it's just counted as minutes instead of data
If you have a surplus of 'phone display data'
You want to use that instead of the limited hotspot data
If you have a limited data total, instead of phone display surplus
Going via cable does not help you
More and more phone plans have large to near unlimited 'on phone data allowance' or some kind of video service included
And limited hotspot data allowance
So sending the phone display to a larger screen aka TV, avoids using the hotspot data, it still uses data just not the limited hotspot allowance
In brief , cabling may allow you to shift the data use to the part of your plan that is not so limited
Depends on the plan and what you are cabling
