Yep, I was using factory.img all along for all versions of 21.02.x, but keeps boot looping. Just tried again tftp method with the latest openwrt-21.02.2-ath79-nand-netgear_wndr3700-v4-squashfs-factory.img, but same result.
tftp method works just fine for openwrt-19.07.9-ar71xx-nand-wndr3700v4-ubi-factory.img