* Use a 3rd party service like ReplIt (https://replit.com) or Code Anywhere (https://codeanywhere.com/pricing). These are fine, and have native iOS apps used to connect to their services as well.
* Pay for a cloud instance and use an SSH tool to connect to it.
The downside to these two options (depending on your needs) is that you are pretty much just targeting Linux and building CLI apps. But that may be totally fine for you.
Personally, I pay $60/year for SplashTop (https://www.splashtop.com), setup a SplashTop server on my home computer, and then use the iOS client to connect to the my home computer and continue programming. And I get the benefit of being able to do other things as well (like playing Steam games) if I want.
Edit: similar to SplashTop, you could also - if interested in Windows - pay for a Shadow PC (https://shadow.tech) and connect to that.