How to Sign in to Your Google Account Using a Mod APK:
1. Download the APK from PS (original) and Google Play Games.
2. Open the game and reach the lobby (home screen).
3. Exit the game, then go to File > Android > Data > "com.ondi.fortias.saga". Rename the file to "com.ondi.fortias.saga00".
4. Delete the APK from PS (original) and then install the mod/cheat.
5. Go to File > Android > Data, then find the previous file and rename it back to its original file name, "com.ondi.fortias.saga00" > "com.ondi.fortias.saga".
6. Open the APK, grant permissions, then immediately exit the game (don't play yet).
7. Go to Settings > App Details > Select Fortias Saga > Delete all data from the game "Fortias Saga".
8. After that, you can play and sign in to your Google account.
I've tried this method and it worked (August 17, 2025).
*My Device: Realme GT6, Android 15, Snapdragon 8s Gen 3 (Non-Rooted)
Apps I used: ZArchiver, GPlayGames, the original app, and the modded app from Platinmod.
(I'm just sharing, maybe someone else has had success like me using this method. If it doesn't work, sorry, I can't help you

).