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...