You'd probably need an earlier POD that established a considerable Christian population in the Chittagong area, too, so that that could be included in your prospective north-eastern state to keep it from being land-locked: Converting Assam as well, so that it doesn't form a Hindu/Indian protrusion into that region, would be an additional help...
(Perhaps keep Assam as a princely state, rather than integrated into British India, and have it princes convert then encourage their subjects to do likewise?; Perhaps also have the Kingdom of Mrauk U, in the Arakan, survive & convert similarly and then be left with India when Burma was split off again in the late 1930s?) A new nation with
those boundaries would be comparable in size to OTL East Bengal/Bangladesh... and if there was still an East Pakistan/Bangladesh ITTL then it might well lack not only the Chittagong Hill Tracts but also the [formerly part of Assam] Sylhet district as well.
EDIT: Maybe a dynastic marriage results in Assam & Mrauk U being held in personal union by a single line of monarchs, or maybe they retain separate kings who share power with one or more other local rulers (Manipur, most probably, and perhaps Tripura too) in either a rotating monarchy (c.f. OTL Malaysia) or a 'Ruling Council'? Being ruled by "Three Kings" (Assam, Mrauk U, and then the other two -- whose individual realms are smaller & poorer -- alternating?) does sound nicely Christian, after all...