Cannot upgrade Netgear WNDR3800

I have a Netgear WNDR3800, running OpenWrt 19.07.3. I want to upgrade it to the latest version (22.03.3).
SysUpgrade does not work. When I do:

cert_verify: cannot parse cert image check failed.

The uploaded image file does not contain a supported format. Make sure that you choose the generic image format for your platform.

How can I flash a new image?

try going to 21.02 1st.

Use a bootloader.

With the bootloader, do I use the sysupgrade image of the factory image?

It should be the factory as I recall.
Please refer to