If you were using the free version of Lightroom mobile on your iPhone, uninstalling/deleting the app also deletes all locally stored data associated with that app, such as the images and presets. The only way you can get those back is by restoring the phone from a previous iCloud or iTunes backup (if you have one).
If you are using a subscription version of Lightroom mobile, all your data should automatically be restored from the Adobe cloud.
Used the free mobile version Lr. I went to the Adobe website and bought the full version. All photos were deleted after purchase. What to do in this case? I didn't uninstall the app, I just bought the full version.