Mount filesyustem RW,

mount -no remount,rw /

Use vi to edit /scripts/system/clientpref/customization_application_startup.sh and add the lines. This should then look like:

#!/bin/sh
iptables -t mangle -D POSTROUTING -d 172.19.0.0/16 -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 900
exit 0;

Save the changes and exit vi
Write protect the startup script again:
mount -no remount,ro /