You need to be on version 50 or higher. Follow these steps on your phone
Go to your discourse settings page and scroll to the bottom.
Enable push notifications.
You’re done!
You should now get notifications regardless of whether you’re currently on the site in your browser. There is no support for mobile Safari (blame Apple).
Chrome is actually not Safari, it uses WebView which is a crapper version of Safari’s rendering engine, so on iOS the best performance is from the Safari app.
I think Apple hasn’t and won’t implement this because it would drain battery, push notifications would be better.