OneAssist Consumer Solutions — mobile replacement certificate not sent

I had given my iPhone x for repair in July 2018 to one assist.but one assist replaced my phone and gave me a new replaced iPhone but they didn't sent the replacement certificate or iPhone swapping certificate.
Now when I am trying to sell my iPhone, nobody is buying it because the IMEI number on the box and phone are not matching.
I have dozens of emails to the one assist customer care and also made hundreds of calls, but they always give the same excuse that this request is given to the backend team and they don't have any update.
Now I am stuck in nowhere.my phones value is getting depriciated as my warranty to going to end very soon and without this replacement certificate nobody will buy this phone.
I have given 10000 rupees when I opted for the one assist insurance service but this unprofessional behavior shows that this company is a cheat company as they charge money for repairs and replace the phone without informing the customer.