How Can I Turn On Heated Seats With My Phone?

I would like to turn on my car's heated seats over a mobile/cellular network using my iPhone, and I'm trying to figure out the simplest path. I know very little beyond that goal, but I know that RF, line-of-sight, Bluetooth, and WiFi solutions won't cut it. I think the amount of data would be on the order of 1MB per month—I'm not about to pay Verizon/AT&T $50/mo for this. The only other constraint is extremely low power use when idle (tens of milliamps, I guess) and maybe a latency limit of like 10s.

Here are the avenues I intend to pursue, and I'm hoping someone around here might be able to steer me in a good direction:

1) Buy something. Is there a thing somewhere that has an iOS app where you push a button and it flips a switch half a mile away? I'm aware of SwitchBot and other home automation products, but none seem to operate over a mobile network. I'd also much prefer an electrical solution over mechanical...and since I might use this dumb thing for years, recurring fees matter.

2) Build something. I can program, and I have an Arduino (I've always wanted a reason to mess with a Pi, too). There is a wealth of info here, but I'd love a high-level "this is what you can expect" from someone who knows...I think I'd need to do this:

- Server. Receives the signal (HTTP? Something else??), triggers a 12V relay somewhere for x minutes, and draws little power when "idle". I'd have to program the hardware outputs, network reception, and timing stuff.

- Controller app. I either write my own iPhone app, or maybe there's something out there for controlling Arduinos/Pis, or provides some nice interface for executing remote requests? I'm out of my element here, but essentially I'd like to push a button on my phone, it sends out a signal and displays success acknowledgement.

- Modem. Seems like 5G/LTE is unavailable, but 4G is common? Whatever is cheap, reliable, and unlikely to disappear in the next few years. Arduinos have options here, and I assume Pis have something too...

- Network. Are there, like, super cheap modem plans for this kind of thing or what? Any usable phone plan would be way too much for this.

good luck.. not going to happen. buy a remote starter with a mobile telematics unit, and your lift will be so much easier