Saegzz wrote:
I checked my settings and I have no SIM restrictions. I’m assuming this an issue for older iPhone models
Your iPhone might have no SIM restrictions – but you are not the OP. It is quite possible that the iPhone which the OP is trying to use does.
In general, just because a cell phone is a newer model does not mean that it will be unlocked. It's easier to buy an unlocked phone than it used to be. But if I went to the Apple site, and bought an iPhone 16 series phone with one of the carrier financing options, I'm pretty sure that phone would come locked to that carrier. Likewise if I went to the Verizon Store or the AT&T Store and purchased that iPhone there, with carrier financing.
The OP referred to a "Paid off iPhone 14 Plus" – which indicates that the OP's phone was paid for with a financing plan, probably one from AT&T, implying that the phone would have been locked to AT&T and that the issue is now convincing AT&T to unlock it.