This mod APK requires an internet connection when you first time open it after installing, but after that, you can play it offline without an internet connection. If you are looking for Offline Mode or Playable offline then there is no option to play this game offline, but yes if you don't have WiFi then it is possible to play Plague Inc Mod APK on your android smartphone by following the below guide:
1) After installation of the APK file, open it and go to the settings
2) Then Data Storage
3) Click the "Clear App Data" button
4) Now close the game
5) Go to Settings
6) Click Network
7) Click "WiFi"
8) Turn off WiFi
9) Start playing Whenever you need internet again