If you are trying to use Internet recovery that won’t work as the macOS it is based on is the older version. You’ll need to swap it out with your original so you can setup a USB thumb drive as an OS installer using your systems Big Sur OS installer. First format the thumb drive to GUID Extended Mac. then follow this guide [https://support.apple.com/en-us/HT201372|Create a bootable installer for macOS]
Now swap over the SSD again and then use the installer thumb drive to first format the new drive and install the OS. The last step is migrating your stuff back from your backup.