The way that I am reading the original poster's message is that they believe that the hotspot feature in Windows 10 is that the computer can literally be a hotspot, including the data connection, but it isn't and can't.
The Mobile Hotspot feature of Windows 10 allows the machine to act as a virtual WiFi hotspot but using the data connection it already has to something else, like it's own WiFi connection, ethernet connection, USB tethering to an access point, etc. Essentially it becomes a router, but not "modem-router" since the modem part of that type of device is the data connection part.
The closest thing to what's being asked for would be if one had a wireless internet dongle and wanted to share its internet connectivity using the computer as the virtual hotspot feature to act as the WiFi router for other devices to connect through it.
If you're out of data on the phone you use as your usual wireless access point for your computer then you're going to have to buy more data or find another way to connect to the internet until your next billing/data cycle occurs.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
. . . the presumption of innocence, while essential in the legal realm, does not mean the elimination of common sense outside it. The willing suspension of disbelief has its limits, or should.
~ Ruth Marcus, November 10, 2017, in Washington Post article, Bannon is right: It’s no coincidence The Post broke the Moore story