App notification fix for IOS and Android

The App has been made a few years ago but the notification doesn't work, which is highly important since this is a staffing app. A worker creates a profile and a business owner logs on the app and picks from the available workers. We pay the worker through the app and receive payment from the business through the app. This app has been made on the web, IOS, and Android. I am looking for an estimate to fix the notification and make sure the app works for budgeting to pick up this app and launch it on IOS, Android, and the web. I am also looking to know the cost for ongoing service of the app for updates, support, bug fixes, and general upkeep for the app.