Zyxel NSA 325 / Restore stock firmware (no replies)

Hello,

I am currently using OpenWrt on my NSA325 but I would like to run the stock firmware ( V4.81(AALS.1)C0 which is available from official page https://www.zyxel.com/support/SupportLandingSR.shtml?c=gb&l=en&kbid=M-01820&md=NSA325%20v2 ) to try to set Power On After Power Failure ( https://forum.doozan.com/read.php?3,92579 ). But I have no clue how to corectly reflash it.

Does someone know how to do it?

Thanks,

Tomas