I am not a historian, but here's what I've found from a cursory look at the literature.
There's Pre-Aksumite, but that demonym isn't exclusive to D'MT.
From "Punt and Aksum: Egypt and the Horn of Africa.":
There are extensive remains of a Pre-Aksumite culture (that is, the kingdom
of D'MT in particular) in the area surrounding Aksum, although little has
been excavated until very recently at Aksum itself.
From "The First Millennium BC in the Highlands of Northern Ethiopia and South-Central Eritrea: A Reassessment of Cultural and Political Development.":
Similarly, it is argued that
interpretation of the epigraphic evidence as indicating a single 'Pre-Aksumite' state
called D'MT is unjustified.
This article argues that there wasn't a single Pre-Aksumite kingdom and even goes so far as to speculate that D'MT might not have even been a state. So I'm not sure how appropriate Pre-Aksumite is as a demonym for D’MT.
There's also the option of just using D'MT as a demonym.
From "Punt and Aksum: Egypt and the Horn of Africa.":
At this
time, a written language was introduced into the D'MT area that seems
almost entirely Saba'an in origin, monumental buildings were first
constructed in ashlar masonry, and large-scale sculpture was produced.
From "The Development of Ancient States in the Northern Horn of Africa, C. 3000 BC—AD 1000: An Archaeological Outline.":
In turn, the Sabean title mukarib (mkrb), used in the inscriptions of a few D'MT kings,
suggests that the pre-Aksumite state was organized on the Sabean model, but the possible
role of the queens may reflect a local tradition and support an indigenous origin of the
D'MT polity (Schneider 1976).
I would recommend using D'MT as the demonym since it's more specific, and seems to be used more, unless you're discussing the wider Pre-Aksumite culture.
Sources:
Fattovich, Rodolfo. "The Development of Ancient States in the Northern Horn of Africa, C. 3000 BC—AD 1000: An Archaeological Outline." Journal of World Prehistory 23, no. 3 (2010): 145-75. http://www.jstor.org/stable/25801293.
Phillips, Jacke. "Punt and Aksum: Egypt and the Horn of Africa." The Journal of African History 38, no. 3 (1997): 423-57. http://www.jstor.org/stable/182543.
Phillipson, David W. "The First Millennium BC in the Highlands of Northern Ethiopia and South-Central Eritrea: A Reassessment of Cultural and Political Development." The African Archaeological Review 26, no. 4 (2009): 257-74. http://www.jstor.org/stable/40389405.
