You don't have a data limit set on your S21, do you @Pittsy? You can check if you do by going to Settings > Connections > Data usage > Billing cycle and data warning > Click the switch next to 'Set data limit' to enable or disable data limits.
Did anyone find a viable fix for this!? I'm also having this issue. WiFi and data are both slow on my Galaxy S21 Ultra, but work perfectly fine on all other devices. I only bought this phone this week, and have updated to the latest available software version.
Resets don't help permanently, only temporarily. A reset of my network settings seems to resolve the problem until I set my connections back up and switch between them. I find that my Internet speed plummets mainly when switching between WiFi and data, and it then continues to be slow for a long time after. Apps load slower, Internet loads slower, videos and files send/receive slower. This is only on my S21 Ultra. My Oneplus works perfectly on both connections every time.
Utterly unacceptable for a device of this cost, and needs to be rectified immediately. But the fact that I see forum posts like this from months ago is very, very disheartening.