I have experimented with the Particle Electron. It is inexpensive and the cell plan is $3/mo. Built in SIM module and microcontroller. You can run C++ so should be simple to write code to operate a stepper.

There are a few option for using GSM networks with Arduino. Here is a link to one https://banggood.app.link/6E6LVtkHke b" target="true">SIM800L Module Board Quad Band SMS Data GSM

Great Scott made a video and it looks like he used a better quality module. He provided a link to it. Great Scott GSM SMS

If you want it to be web based, the biggest issue I can see is not having a static IP. You might need to have a server that it reports into to get it's IP and be an intermediary. Although you could initiate it by text message to the device and report back it's IP that you could then connect to. This could all be done in the background of the app. Or just keep the controls through SMS.

What is your plan for creating the app? What OS? If you do not know JS or Kotlin, MIT App Inventor is free and you can make a decent app from there for Android.

Edit: I should also note that not all sim cards are the same and not all GSM modules will work in all countries. Make sure you research all that beforehand.

@madmisha HI, and thanks for supporting 🙂 Link is dead, do you have another? And is there a possibility with a data cell subscription instead of SMS? I can handle iOS, Android and web app.

Here is one with an external antenna from Banngood:

https://banggood.app.link/KOeybDykme b" target="true">SIM900 Quad Band GSM GPRS Shield Development Board Geekcreit

It's at least a brand I've seen before and probably had some products by. Here is a smaller one with built in antenna:

https://banggood.app.link/7s2C3WNkme b" target="true">SIM800L ESP-800L GPRS GSM Module Micro SIM Card Core Board

I was a little surprised to find this:

https://banggood.app.link/Qi8UgE1kme b" target="true">2pcs LILYGO® TTGO T-Call V1.3 ESP32 Wireless Module GPRS Antenna SIM Card SIM800L Board

It's a bit more money but it is an ESP32 with the GSM module built in.

Unfortunately, because as far as I know, you cannot get a static IP from a cell network, you would need some way to tell your phone or browser were to go to connect. Either the device would need to go online to a known place(server, a Google drive etc.) and report what it's current IP is or have a server that does everything and have your devices report to it. I have not experimented with this so I am not even sure you could connect to a phone even when you know it's IP. They are dynamically assigned and may get assigned for every connection. Considering the number of phones out there, carriers might be doing some type of forwarding to accomplish this and only have a few IP's that is shared. Or the IP is a local IP to your network.

Edit: I seem to be having some issues with links here. If you search Banngood, you should find a few options.

Although, if you can initiate some sort of stream between the 2 it might stay open. Might also eat up your data plan. Getting it to connect would still need an intermediary or SMS, MMS etc.