iphone se locked to three

Hi,

I'm having real issues with getting my iphone SE 1st Gen unlocked from Three.

The phone was bought from Apple as an unlocked device, and has had about 3 or 4 different carriers sims installed since 2017. But when trying to change from Three to another network, it states that any sim installed is Invalid (unless I use a Three sim).

Three tell me the phone is unlocked, but Apple tell me it's locked to Three. That's where I am.

I have tried erasing the phone, setting up as a new device through itunes, resetting network settings, etc but nothing is working.

The last conversation I had with an apple level 2 tech was that the phone is definitrly locked to the Three network. But three are adamant it is not locked to them.

Interestlingly when I upgraded to IOS14 yesterday, there is a new field in the Setting->General->about-> area called "Network Provider Lock" and my phone states "SIM locked" in here, and when I click on that it states;

This device might be restricted to the original network provider and/or have certain capabilities limited. Contact your provider for more info

I think this makes it point more towards a three issue?

I am slowly losing my mind because of this. Can anyone offer any ideas or advice to resolve this?