Muhammad Bakhtiyar Khilji led his army into Bihar and Bengal. In AH601 (1204/05 CE) the city and district of Gaur (Lakhnavati) in western Bengal was captured and a Muslim government was set up in the name of Ghurid sultan.
Gaur/Gour refers to Lakhnauti / Lakhnavati, the ruined city located on the India-Bangladesh border. Most of the former citadel is located in the present-day Malda district of West Bengal, India, while a smaller part is located in Chapainawabganj District of Bangladesh.
Shams al-Din Ilyas moved his capital from Lakhnauti to Pandua, which was renamed Firuzabad. In AH756 (1355 CE) he signed a treaty with Firuz III of Delhi by which Bengal was formally recognised as an independent kingdom.
Pandua, also historically known as Hazrat Pandua and later Firuzabad, is a ruined city in the Malda district of the Indian state of West Bengal. It served as the capital city of the independent Sultanate of Bengal for nearly a century, until the capital was moved to nearby Lakhnauti in 1450.
So it is incorrect to state that the Bengal sultanate “comprising most of the lands of modern day Bangladesh”. The capital of the independent sultanate was in present day India, the lands of the sultanate were largely in present day India (see map of Bengal Sultanate)