This cache is not at the listed coordinates. You must first complete Tealby Relents Adventure Lab, which will take you on a loop around Isaacs Ridge, hence the terrain rating for this bonus cache. The final point on the Adventure will give you 6 digits, which will lead you to the first waypoint in a ring of stones. Open the door to find 6 more digits to get to GZ and a large ammo box a short distance away.
A prize awaits for FTF. Congratulations to: