Phase 3 can't access Red Sands?

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I'm confused. I'm in Phase 3, and was heading to Blackfell, but hit a wall I can't pass. It explicitly says on Phase screen that phase 3 has "all areas unlocked"... if that is the case, why can't I access Red Sands?

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I was able to get it to try to load the location, but it very briefly mentioned something about "frontier pass"?

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After getting that message a few times, it seems we have to unlock a higher difficulty Frontier Pass by playing prime wars.

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Okay, I figured out what happened. When heading to Blackfell for first time, I took a road that passed by a prime zone, and that is why it blocked me. I thought it was all Red Sands, but it was just that route. Made it very confusing since map was grayed out in that area.

Hopefully this helps someone in future, because I couldn't find anything on what was happening.