The D3400's ISO range technically goes up to 25,600, but in real-world use it stays genuinely clean through about ISO 800-1600, gets noticeably grainy by 3200, and is rough beyond that. Every setting below is built around that limit — keeping ISO as low as the situation allows, and using shutter speed, aperture, and the tripod to make up the difference instead.
A narrow aperture plus the CPL filter (which also cuts 1-2 stops of light) slows your shutter down naturally, giving that soft, silky water look without needing an ND filter. Mount on the tripod — at these shutter speeds, handheld will blur everything, not just the water. If the shot still isn't slow enough in bright daylight, that's the one case where waiting for a cloud to pass over genuinely helps.
Ice and snow trick your camera's meter into underexposing — it sees all that bright white and tries to darken it toward gray. Dialing in positive exposure compensation corrects this and keeps the ice looking genuinely white and bright rather than dull. The CPL cuts glare off wet ice and deepens the blue tones inside the icebergs — this is exactly the situation that filter was bought for.
Focus is the part people get wrong: autofocus doesn't work in the dark. Switch the lens to manual focus (MF), then either focus on a distant light before it gets fully dark and don't touch the ring again, or use Live View, zoom in digitally on a bright star, and turn the focus ring until it's a sharp point rather than a blur. Start with a longer shutter (15s) for faint, slow-moving aurora; if it's bright and moving fast, drop to 5-8 seconds or you'll lose the ribbon-like detail to blur. Tripod is mandatory here — this isn't a "helps" situation, it's a "won't work at all without one" situation. Shoot RAW if your card has room — it gives real recovery latitude in editing that JPEG doesn't.
This range keeps both foreground and background sharp — the workhorse setting for most of the trip. Leave the CPL on and rotate it while looking through the viewfinder; you'll see the sky and water darken/lighten as you turn it. Stop rotating at the point that looks best to your eye, not a fixed setting — it changes with your angle to the sun.