The battery adhesive of the Samsung Galaxy S21 Ultra is very strong. If you can't manage to remove the battery by only using high isopropyl alcohol (over 90%) you can try a combination with heat. In this case prepare an iOpener and apply it to the screen for at least two minutes to loosen the adhesive underneath the battery.

Apply 1-2 drops of high concentration (over 90%) isopropyl alcohol to the gaps in between the frame and the left and right edge of the battery.

Only apply a few drops and come back again if necessary. The are two cutouts in the midframe of the phone and too much isopropyl alcohol may run to the screen and loosen or damage it.

Let the isopropyl alcohol penetrate the battery adhesive for at least two to three minutes.