Use APKPure App
Get Chat Unlimited old version APK for Android
Enjoy with offline chat.
As its name signify that its not bound to any limitations like internet connection, any contacts.
Its only require Bluetooth connectivity for chatting with your friends if they are near to you.
You can use this app many places like:
classroom, shared rooms, examination halls and many more.
One important feature I introduced that you can see your conversation history separately from your current conversation.