Sanniquellie is a long day's road journey from Monrovia — around 380 km via Gbarnga and Ganta. The first part of the road is in reasonable shape; the last stretch beyond Ganta is rougher and slower, especially in the rainy season (May–October).
Shared taxis run from Monrovia's Red Light junction to Ganta throughout the morning, with onward shared transport to Sanniquellie from there. The trip will typically eat the whole day. A 4x4 with driver is the comfortable option if you can afford it. There is no scheduled air service to Sanniquellie or Nimba Airport at present.
For crossings to Guinea, the border at Lola is reachable from Yekepa, but visa formalities should be sorted out before arrival.