1) In the DDWRT GUI go to “Administration”
2) Click “commands”
3) Paste this codes in the white command box (-n means newline):

stopservice pptpd && startservice pptpd

echo -n ‘iptables -I FORWARD -p tcp –tcp-flags SYN,RST SYN -j TCPMSS –clamp-mss-to-pmtu’ >> /tmp/pptpd/ip-up

4) Choose “Save Startup”.