Is there a way to change the PON serial of a PM7300-T0 ?

mtest001

I am looking for a way to change the PON serial of my PM7300-T0, to spoof the serial of the router provided by my ISP (Swisscom).

It seems that the command "sys atgs" is precisely made for that, but unfortunately it returns an error message that says "Access denied".

I am now trying to go low-level access through the serial console port on the PCB, but not sure if this is going to change anything.

Any advice would be welcome.