Make sure to follow the how-to carefully:
- Install the NAT helper package
- Enable the conntrack helper
- Enable the firewall service
https://openwrt.org/docs/guide-user/services/vpn/pptp/client#instructions
If the issue persists, try to decrease MTU:
PPTP client MTU value problem - #56 by vgaetera
Why is there a public IP on the VPN interface?