Upgrade NBG6716 to 21.02.0 without TFTP?

Attempting to upgrade from any previous version isn't working, unless I resort to using TFTP-recovery. It works, but it's a clunky and cumbersome method.

Attempting to use sysupgrade just causes OpenWRT to complain it's an invalid image and ignoring the complaint and proceeding with the upgrade causes the router to semi-brick -- it doesn't even attempt to boot afterwards, with no LED flashing or anything. Attempting to flash the factory-image directly with mtd -r write to /dev/mtd7 just causes the router to go into a permanent boot loop.

Is there some method of getting the upgrade to work without resorting to TFTP?

I am also interested by an answer... Got no issue to update all the version in the 19 branch but "image failed" warning while trying to update to 21.02 version.
It is may-be linked to firmware moving from ar71xx/generic to ath79/generic platform??
May-be we have to wait for 21 version beeing stabilized??

Any hint welcome.

What sort of "stabilization" are you expecting, exactly? I have two different revisions of the NBG6716, one with 128MB NAND and one with 256MB NAND, and the 21-series firmware is perfectly stable on both -- I am not seeing any issues with the 21-series compared to previous ones. It's just the upgrading that's b0rked.

I am precisely talking about upgrade process…
(i have the NBG6616)

I mean how to update to 21-series... seems that people that force installation despite the warning message got the router bricked

I wouldn't say it's bricked, if you can just reflash it via TFTP. It's more of a nuisance.