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.
Brings 5G Nationwide access to your device! 5G Nationwide is available in 1,800+ cities. You'll see a 5G icon when you're in a 5G Nationwide coverage area. For more information about 5G Nationwide, see our5G Mobile FAQs