Do you have one of the Adobe subscription plans, or are you using the free version of Lightroom Mobile? If you have a subscription, simply re-install the app from the App Store and login using the details associated with your subscription. That should automatically restore your images from the Adobe cloud.
However, if you were using the free version of LrMobile, you need to be aware that syncing is not part of that version, so there is no automatic backup of those images made by Adobe. The user is responsible for their own backups, such as using iCloud or iTunes to backup the complete phone.