If you are having an issue with precisely connecting your iPhone to 5G, follow these particular steps:
1. Go to your cellular or phone settings
2. Select Cellular option
3. Now click on cellular data
4. Choose the cellular data of your choice (in your case, 5G)
5. Apply changes, and you are done.
You can also check how much data is used by going to the network & internet under the settings option and opening the data usage tab.