Just a note on configuring OpenWRT and PPTP VPN.

Install:

kmod-nf-nathelper 
kmod-nf-nathelper-extra

NOTE: Those will be removed when you upgrade OpenWRT trough Sysupgrade image, so be sure to install them again after an upgrade.

If this does not fix the issue with PPTP VPN’s not connecting then ssh to the router and create a file: /etc/nftables.d/20-pptp-conntrack-helper.nft with the following content:

chain user_pre_output {
    type filter hook output priority -1; policy accept;
    tcp dport 1723 ct helper set "pptp"
}

and reboot the router.
https://github.com/openwrt/openwrt/issues/13009

P.S: If for some reason OpenWRT forgot to load your firewall rules after sysupgrade

fw4 print > /tmp/fw4.rules
nft -f /tmp/fw4.rules
/etc/init.d/firewall enable
ls -l /etc/rc.d/*firewall*