In your community, there are channels that can be joined but are not visible by default. These are called non-default channels.
Currently, Heartbeat only allows joining a non-default channel on desktop and not on mobile.
How to join a non-default channel
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Click on the 3-dot menu at the top of the channels list and select "View all channels".
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Once the desired channel is opened, Heartbeat will provide an option to join the channel.