No, you need to be disconnected from your Stratus or Sentry in order to use any function that requires an internet connection, such as filing or activating a flight plan. When a Stratus or Sentry is connected to an iPad or iPhone, the app can not receive internet data via cellular.
If you have a device with cellular, to file or activate a flight plan:
1. Swipe-down from the top-right of the screen to reveal the iOS/iPadOS control panel then,
2. Tap the "Wi-Fi" button to turn Wi-Fi OFF (button turns white)
Wi-Fi is OFF when the button shows white:
With Wi-Fi OFF your device can use cellular data to file or activate a flight plan.
To re-activate Wi-Fi, swipe-down from the top-right of the screen to reveal the iOS/iPadOS control panel, then tap the Wi-Fi button (so it turns blue) to turn Wi-Fi ON. Your iPad or iPhone should then automatically reconnect to your Sentry, Stratus, or other Wi-Fi network.