oh it's certainly a neurotransmitter, binding metabotropic (G-coupled) receptors, whether endogenous or exogenous; this isn't really disputable. it's a competitive ligand at active binding sites, and thus initiates signal transduction as such.

what is its function?

well, that is a question on par with, "what is the function of consciousness?"

since it is rapidly metabolized by MAO and isn't normally present in any active concentration, it obviously doesn't have any vital function.

dmt production inhibits serotonin production, thus inhibiting melatonin production; dmt is a side-reaction product.