Cameroon was on the other side in the Great War, by the time of the sequel there were three Cameroons (sort of). The victorious Brits and French divided German Kamerun between them. The Brits got two non-contiguous parts, Northern Cameroons and Southern Cameroons (Known collectively as British Cameroons) while the French got the bulk of the colony, it becoming the French mandate of Cameroun.
Interestingly, the Belgians, who also participated in yanking Kamerun from Germany's grasp, didn't get anything for their trouble.
Due to British Cameroons both having a border with Nigeria I suppose it is perfectly possible that Nigeria recruited from there. That is mere speculation, mind, I have no solid info on that.
When independence rolled around plebiscites were held, with Southern Cameroons opting to join the Republic of Cameroon, while the people of Northern Cameroons decided they wanted to be Nigerians, which I think they still are.