Disable 5G on iPhone SE 3

You can see what options your carrier has allowed under Settings>Cellular>Cellular Data Options. My carrier, Verizon, gives me the option of using LTE instead of 5G. Your carrier may have different options.

Note: depending on your country, it may say "Mobile" rather than Cellular.

If there is an option it should be somewhere in Settings / Cellular Data (Mobile Data or Mobile Service in some regions). However, you will have to explore for yourself because many of the options in this section can be visible or hidden by your network's choice.