How to Enable Passthrough for PPTP Traffic - LEDE - OpenWrt

OpenWrt SNAPSHOT r5986-3f7908d / LuCI Master (git-18.031.70145-518ac52)

2. Install the packages

opkg update
opkg install kmod-nf-nathelper-extra     (ppp-mod-pptp, kmod-pptp, luci-proto-ppp)

3. Add a line to the file /etc/sysctl.d/local.conf
   net.netfilter.nf_conntrack_helper = 1
   and reboot router

PPTP Passthrough works !