Multi-device push notifications are supported without need for additional configuration. However, an important condition to note is that ALL devices must be offline in order for push notifications to be sent.
To check whether a user is online or offline, use the SendBird Dashboard. Under the Users tab, each user has an indicator displaying their connection status. When a user is offline here, push notifications will be broadcasted.
One common mistake is for developers have an emulator open with an active connection to SendBird - push notifications will not be sent to any device in this case!