The feed is signed (usign, fingerprint 807479500e0ce219). Install the public key once, then add the feed:
wget -O /etc/opkg/keys/807479500e0ce219 https://roman-png.github.io/VPNpool/vpnpool-feed.pub echo "src/gz vpnpool https://roman-png.github.io/VPNpool" >> /etc/opkg/customfeeds.conf opkg update opkg install luci-app-vpnpool