Why the S21 get disconnected everytime from the wifi ?
if you can, reproduce the issue then submit error report to Samsung Members app.
The techs there should be able to look deeping into the issue and see if it's a software related or hardware related issue and guide you accordingly.
Could you share the device's model number and the software version? thanks
If you can go to Settings->Connections->Wifi->3 dot menu upper right->Advanced->Switch to mobile data=toggle OFF
If it was on this may solve your issue.
Galaxy S21 Ultra 512g(VZW), Galaxy Book (original, VZW), Tab S4, Galaxy Watch3 (VZW)
Hey there, were you able to give the suggestions here a try with any success?
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I am very sorry to hear of the S21 disconnecting from Wi-Fi, and can see how this is frustrating when trying to surf the web or make use of Wi-Fi calling. Sometimes Wi-Fi problems are caused by simple network issues. Other times, you may need to take additional steps on your phone or tablet to ensure your Wi-Fi connection is stable, such as re-adding the connection or assigning a static IP address. Give these steps a try: https://www.samsung.com/us/support/troubleshooting/TSG01001559/
Also, you can see if a third-party application might be to blame with Safe Mode. The guide for that is here: https://www.samsung.com/us/support/answer/ANS00062983/
Please let me know if this helps.