There's two ways that I know of to get through the mountain range...
1- If you have the teleport spell and the location marked on your map, it'll bring you there without having to walk.
2- To the north, a bit west of Stillwater, is Hardin's Pass. You can cross the mountains there. If you want a precise location, look below
Have fun
Slight Spoiler:
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The pass is at coordinates 1288 by 713