My iPhone SE is not reading my sim
How do I get my device to connect to my current carrier… at the moment I’m just purchased on fb marketplace and the iPhone originally purchased from a different carrier than I use
iPhone SE, iOS 16
If changing cellular carrier is really your only issue:
How to unlock your iPhone for use with a different carrier - Apple Support
More likely, your iPhone is Activation Locked because the previous owner did not correctly prepare the iPhone for sale. If that’s the case, read and follow the article linked above your post. You need the original owner’s assistance or the iPhone will remain useless forever.