OK this has given me the biggest headache my phone after update didn't allow calls or texts anything but ee works calls text data how strange is that omg
Sadly for the OP (who has sent the IMEI to me via DM to check), the device is indeed blacklisted.
They'll be reporting this to the seller and hopefully get a refund.
Please watch out when buying second hand as there are a lot of bad actors out there who play the system to get the insurance money and the sale money leaving the buyer to pick up the pieces.