While waiting for a fix from the team, you can try to reinstall the game completely, it should fix the issue.
Please be careful and verify that you are connected to Gamecenter and iCloud before doing so, so your progress is safe.
In order to be connected to iCloud, you can do the following: Go to your device settings and scroll down until you see “iCloud”. There, tap on Apple ID. If you already have an account, simply log in. If you don’t have an account yet, create your own new Apple ID. Enter Royal Revolt 2. The game should now force a iCloud pop-up. Please follow the instructions on the screen to connect your account to iCloud. Also be sure to enter the Royal Revolt 2 settings menu and tick the checkbox next the cloud symbol. From now on, your game progress will be saved automatically via iCloud.
You will just have to play through the first battle of the tutorial before being prompted to recover your account.