How can I disable 5G on my iPhone 13 mini?

Settings->Cellular->Cellular Data Options->Voice & Data->select LTE. That turns off 5G and forces 4G LTE to be used at a times.

Note, that the option to switch to a lower network type as explained by lkrupp, will depend on your specific service provider. Not all carriers and providers allow this, so the option may not appear there.

In general the iPhone should automatically switch to a lower network type if the existing one is too weak.

You're welcome. Glad it worked out.