Gotify.net - free and open-source simple server for sending and receiving messages on Android and desktop. For iOS push notifications, there's no simple and free solution.
For iOS/iPhone there's no free solution available but there's a pretty cheap and nice Pushsafer.com service made by Germans. It costs €0.20- €0.99 per 1000 push notifications depending on monthly volume. Or Pushcut from $1.50/month or $55 for a lifetime.
🔧🔧🔧 = usually few hours of work