There's no 5g option in network mode settings for galaxy s20+
Correct. There is not one. I know some people are looking for it to turn it off (hoping to save battery), but it's not an option too. If you are in 5G coverage area and your carrier supports it, it connects automatically. And if not, the phone knows there is no 5G to connect to and it's not wasting any battery on 5G antennas.