Mirror the feed

Sync the feed with rsync and serve it over HTTP(S). Packages and indexes are signed; routers verify them with the same key.

Sync

rsync -av --delete-delay --delay-updates \
    rsync://openwrt.daeuniverse.org/openwrt-daeuniverse/ \
    /srv/mirror/openwrt-daeuniverse/

--delay-updates and --delete-delay keep indexes consistent with packages. Syncing every few hours is enough.

Serve the directory unchanged; routers request <series>/packages/<arch>/ and <series>/kmods/<release>/<target>/<subtarget>/.

Follow the guide with the mirror in FEED:

FEED=https://mirror.example.org/openwrt-daeuniverse/${DISTRIB_RELEASE%.*}

Import the signing key from this site, not from the mirror.