To unlock a 2nd-hand device you need to call Customer Services on +44 800 956 6000 (option 2, option 4) to put you in touch with the Unlocking Team to request an Unlock Code. You will be asked for your email addy. They will take payment (of £8.99) from any valid UK bank card.
Once the process has been started you will receive an email from the Unlocking Team to process the unlock. It can take up to 10 days to complete the unlock.
But even so beware that EE could still refuse to unlock it. Many devices sold 2nd-hand have been reported lost or stolen, have been sold by the original owner when under 6 months old without EE's permission or have money owing on them.
If that is the case your only remedy is against whoever you got the device from.
If you think I helped please feel free to hit the "Star" button below.
To phone EE: The local rate landline number +44 207 362 0200 or Freephone +44 800 079 8586 - Option 1 for Mobiles; Option 2 for 4G WiFi; Option 3 for Home Broadband & EE TV.ISPs: 1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC => 2014: EE 20 Meg WBC
The procedure I gave you is the only way for your situation.
The online form is only for devices bought directly from EE. No one here is going to do it on your behalf. This is a discussion board
If you think I helped please feel free to hit the "Star" button below.
To phone EE: The local rate landline number +44 207 362 0200 or Freephone +44 800 079 8586 - Option 1 for Mobiles; Option 2 for 4G WiFi; Option 3 for Home Broadband & EE TV.ISPs: 1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC => 2014: EE 20 Meg WBC
Maybe the first but not the second. Why buy a locked device from a foreign country when you haven't discovered how to unlock it?
If you think I helped please feel free to hit the "Star" button below.
To phone EE: The local rate landline number +44 207 362 0200 or Freephone +44 800 079 8586 - Option 1 for Mobiles; Option 2 for 4G WiFi; Option 3 for Home Broadband & EE TV.ISPs: 1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC => 2014: EE 20 Meg WBC
Quelques espoirs! 
If you think I helped please feel free to hit the "Star" button below.
To phone EE: The local rate landline number +44 207 362 0200 or Freephone +44 800 079 8586 - Option 1 for Mobiles; Option 2 for 4G WiFi; Option 3 for Home Broadband & EE TV.ISPs: 1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC => 2014: EE 20 Meg WBC
