Please refer the below steps for sending bulk push notifications to all registered users at a time.
Step 1: Enable 'Add to Push Notify' option for the module where the device tokens are saved.
example. If you am saving device token of registered users in users table and module for that table is users then you need to enable 'Add to Push Notify' option in listing configuration.
Step 2: Then Enable 'Add to Push Notify' option for the device token field in listing field configuration.
Step 3: Now go to 'Send push Notifications' in Admin panel(reference: image_03).
Step 4: In Send push Notifications form select the module in 'Sent To' option and device token field in 'Select Device Token' option(reference: image_04) and enter your push notifications details in form and click send.