As others have mentioned we need more info to be exact, but assuming your "internet cable" means something like CAT5, and assuming you are only using one pair, then you probably have a serious voltage drop at 900mA.
A rough calculation based on a typical CAT5 cable with 24AWG conductors, using the typical loop resistance of 16Ω for 100m. We will estimate the routers load as 12Ω for a max of 1A:
300ft = 91.44m, so loop resistance = 16Ω * (91.44/100) = 14.63Ω
Total circuit resistance = 14.63Ω + 12Ω = 26.63Ω
Circuit current = 12V / 26.63Ω = 451mA
Cable voltage drop = 0.451A * 14.63Ω = 6.59V
Voltage available at router = 0.451A / 12Ω = 5.4V
So, although this is just a rough estimate (ignoring temperature effects, inductance/capacitance. Also we don't know what kind of regulation circuitry the router uses or it's input range) we can see that there is nowhere near enough power getting through to the router.
I would use at least 18AWG or thicker cable, preferably with some shielding and e.g. a ferrite bead. You can try using multiple conductors to lower the resistance. Also placing a largish capacitor (e.g. > 100uF) across the wires at the router end may help a little.