Can't install vanilla Openwrt on GL-AX1800

I'm trying to install stock OpenWRT.

Specifically https://downloads.openwrt.org/snapshots//targets/qualcommax/ipq60xx/openwrt-qualcommax-ipq60xx-glinet_gl-axt1800-squashfs-factory.bin

When I try to use the manufacturer's web UI I am told:

The firmware is not compatible with this device.

When I try to do it with the luci UI I am told:

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.

In both cases the SHA256 matches what the download page said.

What is the right way to do this? (And not brick my device.)

Thanks,

-kb