Android remembers if they dismiss the banner and will not ask them again. iOS will not prompt them at all unless they open in Safari.
For your users they must open the URL directly with Safari on iOS or Chrome on Android or they will not be able to download the app correctly. In Chrome they can press the 3 dots in the top right and select “Add to Home screen”.
On iOS if they press the share button they can then select “Add to Home Screen”.
In both cases they need to be careful not to open the URL in an in app browser (like found in Slack or Twitter apps).