The normal reset procedure is what you say you have already done... switch the radio ON and leaving it SWITCHED ON ( WITHOUT INTERRUPTION) until the display resets.
The important bit is the WITHOUT INTERRUPTION because the timer will reset if interrupted.
If the previous owner has entered far too many wrong codes then the radio may well and truly be fully locked. If so then it will have to be reset by taking it into a Radio Decoding Service to rewrite the code chip inside..
On some radios, the reset time doubles after each wrong code so as
one last try I'd connect the radio to a battery supply on your workbench and leave it switched on ( without interruption)for as long possible to see if the display will eventually reset (after several days perhaps?)
Otherwise a trip to the Decoding Service or buying a replacement radio with a working code will be the only solutions .
Please post back here with any result