Indeed, System-Software tab is missing and URL /cgi-bin/luci/admin/system/opkg is locked out-of-the-box with error 403.
Fixed this by downgrading to latest available luci-app-opkg.ipk via SSH. Here's how-to:
cd /tmp
wget https://downloads.openwrt.org/snapshots/packages/mipsel_24kc/luci/luci-app-opkg_git-20.159.50935-79ace93_all.ipk --no-check-certificate
opkg install luci-app-opkg_git-20.159.50935-79ace93_all.ipk --force-reinstall
Then Logoff/Logon and here it is!
