which phone do you have?

What Phone / Update do you have? I still have the option to select which sim to use on my 2degrees Note 20 Ultra. 
Android 11, 1 Jan 2021 Patch Level

They've just tucked the options away a bit. For example, if you select a contact, press and hold the green dial button and you'll get the "Make calls with" option.

When responding to a SMS, it responds from the number that you received the message on. You can however select the settings then change from the bottom. (In the default Samsung Messages app).

Also the SIM card manager settings are still under the Connections settings.