How do I stop mode?

See -> Use Lost Mode in Find Devices on iCloud.com

Use Lost Mode in Find Devices on iCloud.com – Apple Support (UK)

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Turn off Lost Mode on an iPhone, iPad, iPod touch or Apple Watch

1. In Find Devices on iCloud.com, click All Devices, then select the device that is in Lost Mode.
2. Click Lost Mode, click Stop Lost Mode, then click Stop Lost Mode again.
3. You can also turn off Lost Mode by entering your passcode on the device.

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