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Smartphones are not really built for such weather conditions. If you have to take it, remove battery, store it somewhere warm, but dry. Use phone only when in need, preferably not outside but during stops like overnight camping or lunch break. Most smartphones also provide ultra battery mode which will help a lot if you use it to navigate.
Apart from that you should learn to orienteer with a map and compass. I'm pretty sure you'll find lots of tutorials in your language but the most basics are like this:
> use the compass to find north
> align your map so it's top is facing it
> find your position (look for landmarks and near you to help you)
> determine the azimuth (deviation from north) and distance to your target
> if it's longer hike, find some landmarks inbetween and use them to orient
> set it on your compass according to what you found out
> now you know which way to walk
