Install an older version on a device which is not listed on the older release

Hi folks
I want to use FW3 instead of FW4 in Archer C6 v3 which is only listed in 22.03 version .
So I decided to install an older Openwrt versions like 21.X which uses FW3.
As you know there is no image file for Archer C6 v3 in 21.X version .
Is there any way to install 21 version on Archer c6 v3?

I think that v 22 was the first official release for that device.

But what is the issue with firewall 4?

yes it is.
I want to install Passwall which not support FW4 for now. I got errors with passwall configuration which I googled and shows the source is that openwrt 22 uses FW4.

If you are fine with self-compiling, take a look at the tool chain of v21 and v22 and both repos.
It looks to me, as if at least CPU and Wifichip was already supported on other devices in v21

yet you might miss a bunch of Wifi patches, it has been a while, no idea how good the 21 Wifi driver is in comparison.

Still, it could be that in ideal case that you just need to backport the devicetree config of that router.
But it might be more effort than you are willing to invest, in case you need to debug problems.

It would be easier to replace fw4 with fw3, than backporting new device support to older releases - both are still available.