Change the firmware on a VMG3925-B10C

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Hi I have a VMG3925-B10C which is a Post Office Router (from an ISP) the firmware is V5.13(AAVF.10)T2 and I want to change it to 5.13(AAVF.18)C0 I tried the normal upgrade method but got this error.

so I tried login into the router with an FTP program but it just shows 2 folders as pictured.

So the question is it possible to restore the standard ZYXEL VMG3925-B10C firmware, any help would be appreciated.

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