You can reach the Umpani base camp from the monastery road. There is a point where the road between the monastery and Arnika splits, with a branch going north. Follow that branch to the next section, then find the bridge and cross. That will take you to the camp.
Note that you will have to sign up with the Umpani in order to obtain a rocket launcher. Once you do the assignments, finding Sgt. Rubble should be a little easier. He isn't around until you join the Umpani
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