Then it means you have a 5g signal, it’s just that the data connection that comes with it is very bad / non-existent. Unfortunately, if the signal exists, many phones won’t bother to also check for working data.
I know some phones can do that in order to toggle between cellular and wifi data, but not sure if they apply it to cellular modes too. See if there’s any setting about “smart” connectivity. On my Sony it claims to also switch 5g / 4g as needed.
If all else fails I guess you can use a tool like Tasker to periodically ping an external service and toggle cellular mode when you have no data, but that’s pretty crude.