How to Troubleshoot your iOS issues
Find the app
If an app causes problems and is open, close it and open it again. If it’s unresponsive, force it to quit. Doubleclick the Home button (or swipe up from the bottom and hold a second on iPhone X) to open the app switcher.
Back up to iCloud
Before trying anything else, make a backup of your device. To do so using iCloud, connect to Wi-Fi, then go to Settings > [your name] > iCloud > iCloud Backup and enable this feature. Start a manual backup and let it finish.
Reinstall the app
In the App Store, tap Today in the bottom bar, then your profile image at the top of the page, and then Purchased. Tap Not on this iPhone/ iPad, locate the app in the list of past purchases, and tap the download icon.
Swipe horizontally to find the app, then swipe upwards on its preview to force the app to quit. On iPhone X, hold your finger on an app’s thumbnail until red minus buttons appear, then swipe up on the one you want to quit.
Back up to iTunes
To back up using iTunes instead, open iTunes on your Mac, connect your device, click its icon at the top left, then click Back Up Now. Once the backup is complete, eject the device and then disconnect it from your Mac.
Turn off Find My iPhone/iPad in Settings > [your name] > iCloud. If you have a Watch, unpair it now. Before trying a full factory reset, consider using Settings > General > Reset > Reset All Settings as your next step.
If restarting the app doesn’t fix the problem, shut down your device by holding the Sleep/Wake button and using the slider to turn it off. Once it has shut down, start it as normal and check for updates in the App Store.
Remove the app
Try iOS 11’s ability to manually offload apps, which deletes the app from your device, not its data. If that doesn’t work, hold a finger on the app’s Home screen icon till it jiggles, tap the X that appears on it, then tap Delete.
Erase and restore
Reset your device in Settings > General > Reset > Erase All Content and Settings. Reinstall the app and see if it works. If it does, reset again, restore from your backup and try again. If the issue returns, contact the developer.