Check with your carrier to make sure you have the correct data plan . Also, make sure you are using the SIM card that came with your phone or a 5G compatible SIM provided by your carrier. Older SIM cards may not be compatible with 5G plans. You can check Network mode to make sure that your phone has 5G enabled.
Read moreWhy is my phone not picking up 5G?
Restart your phone or enable and disable Airplane mode : Some network settings may cause your phone to fail to connect to the 5G network within a short period of time. Restart your phone or enable and disable Airplane mode to resolve the issue.
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