I have Tmobile and the old permeant fix for this issue is to set APN protocol to IPv4 or IPv4/IPv6 from the default IPv6.
Here are the Tmobile defaults. https://www.t-mobile.com/support/devices/android/samsung-galaxy-s10/network-apn-and-volte-samsung-ga... You typically cannot change the default APN settings, but you can create a new one with identical settings except for APN protocol. Different providers will have different, but comparable settings.
I first found this tweak years ago and it is still a must even with the S21.