Listen hiccups happen with our phones. Here are a few things you can try to fix this specific issue. In the end they might now work and you may have a bad SIM card and need to call your carrier.
First…. make sure in settings > celluar > LTE is actually turned on. If it is….
Second… turn your phone off and on. I know, I know so simple, but it fixes 75% of issues I run into.
If you still cannot get LTE…
Third…. Go to settings > general > reset > reset network settings… This is what I had to do.
You will have to re-enter wifi passwords etc… but hopefully everything will work and you will be able to connect to LTE.
If none of that works, then do yourself a favor and call your carrier tech support. Usually takes about 15 minutes and they can make sure the overall network is running fine and if your SIM card might be whacked out.
Hope that helps!