Hey Yan,
I think there are some confusions here, actually that is not our app not support Safari browser and iOS iPhone device, it is currently not support these from the OneSignal itself, you can check their documentation here
https://documentation.onesignal.com/docs/web-push-overview , they got show which browser or OS can be support at this point of time.
If you still unable to setup to allow user to subscribe this notification on Chrome browser, it might be related with this known issue as described from the OneSignal developer
https://stackideas.com/forums/onesignal-box#reply-456418 , we can assist you on this if currently you would like to make it working on the chrome browser.
And Currently I am not really sure about this
jomsocial - plugin - jsannouncer how it work, if this is the one you referring that Jomsocial web push notification plugins
https://www.jomsocial.com/addons/apps/joompush-jomsocial-plugin , if they plugin also rely on this OneSignal platform, mean it would be face the same issue which is unable to support Safari and iOS device.
If you still decide to request refund for this Web Push Notifications app, you can contact us at
https://crm.stackideas.com .