I don't know who CEX is, but unless they're an authorized retailer of Pioneer DJ gear, they're unlikely to be able to assist you from their end in resetting the license.
People are legally allowed to sell the hardware without selling the software; they are technically two separate "items" and can be used individually. If the product you purchased said that a rekordbox license was included, your grievance is with the seller. If they didn't mention it coming with a rekordbox license, the choice is yours to purchase one from rekordbox.com or use another MIDI-compatible software.
Who did you speak with at Pioneer DJ / where did you inquire to try and resolve the issue?