Apple refusing to unlock my icloud

I purchased a second hand iPhone 7 plus from envirophone shop online on 27 December 2019 and it was unlocked. I setup an iCloud account and the iPhone was set up from scratch as new. My phone fall in the floor and it was sent for repair but I am assuming that someone interfered with the iCloud account where the iCloud account was locked. I remember my username and password but I cannot remember the date of birth entered as it must have beed mistyped. I contacted customer services and I was asked for proof of purchase and that was provided. During a conversation with Apple. I was told that my iphone has gone into recovery mode and I tried it via PC but unsuccessful. I contacted Apple Support and I was then told that they need more information. Now Apple is asking me to contact the reseller to provide me or apple with the details of purchase from the re-seller but the re-seller or the place where I bought it has told me that they will not provide me with the information of their purchase because of data protection.  They want supposably to find the person who bought it from apple.

It is completely unreasonable not to unlock the device since i am the owner of the device and I have proof of purchase. The seller has no interest to contact apple and pass details