I had this same issue when I downloaded BigSur from Catalina. What had helped me with the internet connectivity was, I had to restore my laptop to factory default. Before you do that, make sure everything on your laptop is saved onto a memory stick, and/or backed up. After restoring everything to factory default, and downloading BigSur again, my laptop has been connecting to the internet/wifi smoothly. It's a lot of work, but worth it in the end.
princesspajnra_
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