To make PUSH working you need to open OneSignal account then setup it properly for Web Browsers, Android and iOS[if needed] in OneSignal - then have necessary instructions on how to do it. Then specify OneSignal App ID in UNA and mobile apps.

Also you can refer to more detailed description by Mosi76
https://una.io/cmts.php?sys=bx_forum&id=4009&cmt_id=16180

#mobileapps