NOTE for users that have a Connection Method feature (especially MIUI, Infinix, etc.)
No, you cannot use the Connection Method feature to make the game fully offline while Wi-Fi was on, no matter when you open or close the game.
Of course, it may notify you that the 'game was disconnected to Wi-Fi and Mobile Data'. But unfortunately, the game will force to breakthrough the connection system.
So, yeah. If the mod says Offline, it means you need to go Offline completely. No need to modify the connection.