I have tried to locate your "Cameroon Regiment from WW II", without success.
However, I did locate the Legion of Cameroon, organized 27 Aug 1940, in the West Africa nation of Cameroon, which may or may not be what you are looking for.
which lists " The Man With the Missing Finger, Novel:French...division heroes african colonies Cameroon".
Chapter Four of that book describes the mission and operations of the Legion of Cameroon.
Therein is a most interesting history of the French resistance to German occupation of France, as championed by the various colonies,including Cameroon, in direct opposition to the Vichy government in France.
They were brave men and women, indeed.
I hope that this information helps you in your quest, and that you may envision your late grandfather in this setting.