The reason of creating this thread is to inform S21 Ultra owners or people who are planning to buy. This is my advice to you all not to buy any 3rd party S-pen because It may or may not work on your device and many Stylus Online are made for normal devices which are not compatible with orginal S-pen. So, it's waste of money and time.
So, whats the solution ?
you can buy the note series S-pens which you can get for cheap compared to the big S-pen.The description of Note S-pen may say that S-pen has bluetooth. Don't fall for it because S21 Ultra itself doesn't support bluetooth functions. Why? Because Samsung decided to make it exclusive to Note Series.
Compatible S-pen (s)
Samsung Galaxy Note 3 TO Galaxy Note-20.
Note : In Case if you are buying from 3rd party retailers. Please Check The Model Number Of Item Before Buying Because Most Retailers Sell Us Either **bleep** Product Or Product From Other Countries ( you cannot claim warrantly in case of any damage )
I am currently using Note 3 S-pen and if works just fine. But, not as smooth as Note 20 ultra's S-pen. I have to apply slight force in order to navigate or write a note.