I have had the same issue with my s20 ultra and then tab s7+ after the 5g update. I do not live in a 5g covered area, yet my devices constantly switch from 4g lte to 5g over and over, and when it displays 5g icon I have no data transmission. It has been months now since the updates to these devices and it's still happening.
Below is the update that created the problem for my phone. My tablet doesn't allow me to view previous updates like I can with phone, but it basically stated the same thing. It was just released later than the phone update.
Release date: 10/14/2020
Android Security Patch Level: September 2020
Software Version: QP1A.190711.020.G988USQU1BTJ1
The current software update: