Assuming you have checked with your carrier that there are no network issues in your area, that your LTE signal is strong, and your account properly provisioned for LTE, there are some things you can check on the phone itself.
To get a baseline reference point you could start by running a speed test and recording the results.
The first thing to do is toggle Airplane Mode on and then off again.
If that does not help do a forced restart.
Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the Apple logo.
After that, the next thing to check, if is visible on your phone, is the APN settings.
View and edit your Access Point Name (APN) on your iPhone and iPad - Apple Support
You can also try swapping your sim into a different phone to rule out the sim as a possible cause.