Push notifications to specific users or devices

I have a WEB MVC 5 system in which I use signalR and SqlDependency to be in real time, but this is not the case what I need to implement are push notifications to system administrators when there are changes of interest. Being specific what I need is a mechanism to send push notifications to specific users and if possible, even if they are not connected, that is, even if they are not using the system.

I searched a lot, I was using the push.js library but this does not allow me to do this or I think so.

Any mechanism that can provide that covers my needs or something similar will be received.

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