I use 18.06.2 before and PPtP VPN works fine. After upgraded to 18.06.4 I have to add custom iptables rule in
firewall.user for using PPtP VPN, which is described on document: https://openwrt.org/docs/guide-user/services/vpn/pptp/client
The iptables rule is
iptables -t raw -A OUTPUT -p tcp -m tcp --dport 1723 -j CT --helper pptp
I haved added these packages
ppp-mod-pptp kmod-nf-nathelper-extra luci-proto-ppp kmod-ipt-raw
I have tested if I removed the rule, PPtP VPN will be failed and the error message is "negotation error"
My device is