PPTP dial-up fail on OpenWrt 21.02

Installing and Using OpenWrt
7

I finally found the solution in post
[Solved] Ar71xx Kernel 4.9 PPTP Passthrough not working (TL-WR942N v1).

But the root cause of this issue is that nf_conntrack_helper default is not enable for security issue.
Can check it from post below.
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-4.9.y&id=8f9872be6db939a75a075cd893a47ce5742cee00

And from kernel log, it suggests using CT firewall rule instead of enable nf_conntrack_helper for CT helper.

nf_conntrack: default automatic helper assignment has been turned off for security reasons and CT-based firewall rule not found. Use the iptables CT target to attach helpers instead.

Solution is either one of the three

  1. Add a line to the file /etc/sysctl.d/99-pptp.conf (create this file)
    "net.netfilter.nf_conntrack_helper = 1"
  2. echo 1 > proc/sys/net/netfilter/nf_conntrack_helper
  3. add firewall rule "iptables -I OUTPUT -t raw -p tcp --dport 1723 -j CT --helper pptp" (might be the best)