vmg1312-b10d with tp-link firmware

Hi, I have a Zyxell vmg1312-b10d router which I installed to a customer less than 2 months ago. Now the customer tells me that the router has problems connecting and then I go and verify that the web interface is different from the one I remembered. At no point is the Zyxell brand shown and the firmware installed on the router does not seem to be the original one but it would look like a TP-Link firmware. FTP access also responds with a TP-Link server message. How can I restore the original version? the reset procedure brings me back to the TP-link version. I tried to pass the zyxell firmware via ftp but reverted to 550 permission denied. Even the telnet console seems limited in commands. 
I look for suggestions for a return to the original Zyxell firmware. Thank you

#CPE_February