@musashino205 ISRG Root X1で署名されてるのでクライアント側でなんとかしないといけない感じかもです…
$ :| openssl s_client -connect downloads.openwrt.org:443 -servername downloads.openwrt.org > /dev/null
depth=2 C = US, O = Internet Security Research Group, CN = ISRG Root X1
verify return:1
depth=1 C = US, O = Let's Encrypt, CN = R3
verify return:1
depth=0 CN = downloads.openwrt.org
verify return:1
DONE