I purchased this router second hand online and I didn't know it had specific ISP firmware installed.

I can log on to it OK and change settings etc but, if I do a factory reset,  it reverts back to the ISP logon details.  I don't know what other settings might be ISP specific so I would like to be able to do a proper reset.  I even did a firware update but this still kept all the settings.

I believe this might be possible using the TP-Link Aginet Config software but I don't have access to this.  I also logged a ticket with TP-Link support but they say they can't help which I'm not sure about but have to accept.

Does anyone know if there is a way to revert to the standard factory firmware ? 

Thanks