I'd like to upgrade my Zyxel NBG6716 device to 4M kernel.
It is currently running 21.02.3 (r16554-1d4dea6d4f).
When I try to sysupgrade to the 4M kernel firmware (openwrt-21.02.3-ath79-nand-zyxel_nbg6716-squashfs-sysupgrade-4M-Kernel.bin) I get the following error message:
Image check failed:
Tue Aug 2 13:47:35 CEST 2022 upgrade: Image metadata not present Tue Aug 2 13:47:35 CEST 2022 upgrade: Use sysupgrade -F to override this check when downgrading or flashing to vendor firmware Image check failed.
Can I safely force the upgrade or is there a special method to switch to the new 4M kernel firmware?
If I remember correctly, the kernel size was increased in version 19.07, since then there are 4m kernel versions. You're right, it's unnecessary in current versions. But if 4mb kernel size is insufficient again in the future, you can see it as 5 or 6mb kernel again. Someone needs to edit and remove it, but I guess no one is bothering.
But this didn't work for me either in the past, if I remember correctly this gave me the same error you got when you tried it.
What worked is to set up a TFTP server described in the openwrt toh page for zyxel, rename your FACTORY ath79 OpenWrt firmware file to ras.bin and reboot your router while holding the WPS button. For the specifics please look up the instructions. This maybe needs confirmation from others because I did this 2 years ago (with the v19 ath79 image) so my memory is not so fresh about it but I think that was the method I used back then. You can also unbrick your router using this TFTP method btw.