Briar Mailbox is a helper app for Briar messenger (https://f-droid.org/packages/org.briarproject.briar.android/) that lets you receive encrypted messages from your contacts while Briar is offline.
Next time Briar comes online it will automatically fetch the messages from your Mailbox.
Install the Mailbox app on a spare device, link it with your Briar account, and leave it connected to power and Wi-Fi.