If you have read the guide on "How do I receive mobile notifications", it should be easy for you to understand this. Otherwise, you can check it here.
About desktop notifications, it's also very easy. First, check if your account isn't on "Do Not Disturb"
Set your status on "Online" to receive EVERY notification. I recommend you to disable one by one category and channel you don't want to be notified of. This is the safest way for you to be pinged when there's something interesting popping in one specific channel.
Then, as same as mobile notifications, please check if the channel our the category isn't muted, by right-clicking on it and see if it's not muted or no
If you still don't receive any notifications, check-in your Discord default settings.
Make sure you have enabled the first option. This is the most important thing to do.
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