No doubt the species in this photo belongs to the tribe Cyphicerini. I've never seen Sepiomus. If your identification is correct then Sepiomus is a subject of taxonomic publication and is to be transferred to Cyphicerini.

I don't know what is the valid name. For me it is here a Tropiphorini. I have two species of the genus : Sepiomus parallelus and S. frontalis, the both from Cambodge.