For anyone who wanna login to GPlay without rooted device or unsigned apk then there is the way to do it:
1: Download the original aka playstore apk at anywhere yall can find it
2: Install the original apk like usual (apk+obb)
3: Open the game and login to Gplay account then wait for the game load a bit
4: Tap out to the main screen and open ur Storage or Manager Files. Go to Android/data/com.candysoft.SBZombies < rename this package name folder with any letter yall like (for example: com.candysoft.SBZombiesx)
5: Do the same step as above for the obb file too if yall dont wanna reinstall the obb file again
6: Uninstall the original apk then install the modded apk and DO NOT OPEN THE GAME AFTER INSTALLED
7: Go back to step 4 and 5 as above, repeat exactly everything again but this time, rename the package name back to com.candysoft.SBZombies without x letter
8: Go to Setting > App > 7 billion zombies and grant all permissions also the overlay for mod menu too
9: Open the game with modded version and the game should automatically connect to ur Gplay account
Note: this method is kinda workaround, it wont stored ur Gplay account with the game time to time, if u reboot ur device then u have to redo everything again