Firstly, any decent
marine electronics will take 9V-30V as an input range without a problem. A netgear router will not. One of the big differences between Raymarine's
Seatalk HS switch and a normal Ethernet switch is the the
Raymarine unit has a well designed input
power filter and conditioner that takes the 9V-30V and produces a clean 12V without dips and spikes.
Secondly, I would take a look at the gauge of
power cable you're using between the
batteries and the DSM300 & switch. The full length of a DC circuit is from the
batteries to the device AND back again (I.e. 2x the distance).
It certainly sounds like you have a power issue, and the voltage drop in the line at high amp draw suggests two areas to investigate:
Batteries. Even though they may read 12.7V at rest doesn't mean to say they have much AmpHr capacity left in them. Have you checked thr voltage at the batteries when the fridge is turned on? The voltage at the
battery terminals really shouldn't plummet when thr fridge is turned on, if you have adequate capacity and the batteries are good.
If not the
battery, look at the supply cable size. I would get a length of larger gauge wire (than what you're using) and temporarily run (over the
cabin sole) from the batteries, through a fuse, and to the DSM300 and router directly. A larger gauge wire will carry more
current for a given voltage, so at least you would eliminate the possibility that you have a dangerously undersized wire supplying the DSM300...