Damn that is such a total disappointment, sounds like the author has got some more work to do?
Could any kitizen fix it properly?
No, it appears this thread has confused
spring.
The changed firmware is to make 1508 mtu work while the device is being used as a
modem (as per thread title) in bridge mode. From his previous posts (setting ppp session parameters over telnet on the VMG1312) he is using it as a modem/router, in which case he only needed to set the MTU to 1500 in the web interface. His method of changing the ppp session is of course not persistent, just as changing it on your router by direct commands would not survive a reboot.
Maybe I should edit the OP to make it even clearer we are talking about bridge mode?