If the PI is in your home network and the sensor is not, and the sensor does not act as a server (hard with a basic cell connection) and if you want to use TCP-IP then you need to poke holes in your home network.
If you don’t want to poke holes, whatever is on your home network has to be a client of some server architecture hosted somewhere.
If you are willing to pay a bit of a monthly fee, then you can get the whole server stuff hosted.