Tell the two failures apart

If the notification icon is still there but nothing flows, that's the node or the network. If the notification is gone and the VPN key icon vanished from the status bar, the process was killed.

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Switches to turn off

  • Settings → Apps → v2rayNG → Battery, choose Unrestricted (or "don't optimise").
  • Xiaomi keeps a separate control under Battery & performance → app battery saver, and you also need to lock the app card in Recents.
  • On Huawei set Launch management to manual and enable all three: auto-launch, secondary launch, run in background.

One that's easy to miss

Some phones have "keep Wi-Fi on during sleep" in the advanced Wi-Fi settings — set it to Always. Otherwise Wi-Fi drops with the screen and the VPN goes with it.