Assam that we know of today did not exist back then. Fall of the Kamarupa empire lead to the rise of many small kingdoms like the Chutiya and the Kachari kingdoms in the north and South bank of the Brahmaputra, the Koch kingdom to the west and the Baro-Bhuyans in the middle bordering the Ahoms to the east. The Ahoms and the Koch gradually consolidated territories while the other kingdoms diminished in stature, the Koch kingdom was though, a vassal state of the Mughals.
The western border of the Ahom kingdom extended upto the river Manas and the area beyond the southern boundary was under the occupation of Jaintia, Khasi and Garo tribes. Though I may be wrong about this, but the Ahom kingdom never included Mymensingh.
Also, I hope you know that the Rangpur that you have quoted is a part of the present day Sibsagar town, the seat of power of then Ahoms and not the district in Bangladesh! As I have already said, the Ahoms were not expansionists unlike the Sultanate and Mughals, they fought to defend their territories.