If you don't want to pay, use git sync. Personally I think it's decent, have to deal a lot with merge conflicts (I have 3 devices), and using git on iOS is cumbersome.
Or, you can use the LiveSync add-on (need to setup your own CouchDB instance for it).
LiveSync is pretty good, real time sync is nice, but I rarely had a conflict since switching to it.
You can access the notes from your phone and computer. The app is available for Linux, macOS, and Windows. The mobile apps is also available.
You can write notes offline. It will sync automatically when you are connected to the internet.