Thank you for your clear explanation of the history of this phone. The upshot of it is that you bought it outright from Tesco as a PAYG phone. Tesco don't sell SIM-less phones. All phones purchased from Tesco, whether on contract or PAYG, are locked to Tesco.
Therefore only Tesco can unlock it so that an EE SIM can work in it. So you need to apply to Tesco & only Tesco to get it unlocked. There is nowt that EE CS can do to help you in this matter & no end of contacting them will change anything.
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