I have an S21 Ultra and I also have one of the original Samsung Wireless Charger units.
When I plug the charger in, in lights up.
When I place my S21 Ultra on the device, it states "wireless charging" and a time of up to 4.5 hours.
Left it on charge overnight, S21 Ultra became rather warm but device picked up no charge overnight.
Does the S21 Ultra work on older wireless chargers? I've set the Faster Wireless Charging to on.