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for verizon users, you have to be in 5G area with 5G sim and 5G plan. and most importantly, you have to be close enough to be able to see the tower with your eyes, and must be outdoor.
or, you wont be able to use 5G becuase mm wave is fast but weak. it doesnt travel far and cannot go through the walls.
EDIT: Got 5G working on TJ4. You can use the hidden menu to get 5G working.
Using the Google Phone app (not the built in Samsung dialer), dial: *#*#27663368378#*#*
This will bring up a hidden menu. It's a very finniky app and you have to use the Back option within the app's top-right, tripledot menu to navigate (don't use the Android back button).
Once you are in, use the following menu options to enable 5G:
Step 1: Option 2, 1, 2, 9, 9, 9, 9, 1, 1 -> Set Default LTE Band
Step 2: Option 2, 1, 1, 1, 9, 9, 2 -> NR DSS Enable
Step 3: Option 2, 1, 1, 9, 3 -> NR5G PRUNE is removed
Reboot and you should have 5g.
Step 1 enables all LTE bands which is needed to get 5G working. Step 2 enables Dynamic Spectrum Sharing which allows 5G and 4G LTE to co-exist on the same spectrum. Step 3 enables all 5G bands.