5g on 13

1. Make sure that you’re in an area with 5G coverage. Contact your carrier if you're not sure.
2. Go to Settings > Cellular > Cellular Data Options. If under "Voice and Data" you see 5G, your device has 5G activated. If you don’t see this screen, contact your carrier to confirm that your plan supports 5G.

You need to buy the 5G plan separately. Make sure you've followed all the steps here - Use 5G with your iPhone - Apple Support

Did you contact your cellular carrier? is your plan okay? Did they exchange your SIM card?

Did you perform a forced restart of your 13, as per these instructions?:--> Force restart iPhone – Apple Support

If the issue persists, since your iPhone is still covered by warranty, make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

Hey there,

See ☞ Use 5G with your iPhone - Apple Support