Believe it or not, some 5G networks, specifically AT&T's, are still unreliable. It may show you, the user, that you are on a 5G network, but in reality, you are stuck in some weird cellular limbo. We run into this a lot with folks using the Sidekick app in the field.
There's an easy solution. Revert back to LTE and you will be good to go.
What this video to find out how: Turning off 5G.