I wouldn't have thought the voltage drop would have been that bad, if it is replace it with a 12v and watch the modem fry

The holes should be big enough, with a little bit of modification with a hammer! (NOT THE CO-AX ) Drag the new cable in on the back of a bent coat hanger (NOT A WOOD ONE ). Most of that type pf co-ax has the wires welded to the side of it, so you can flatten the cable slightly to get it through the hole without damaging the co-ax.

Cheers

Cris.