Or they could maintain the same birth rate and use technology that was well understood before anyone in this board was born. Huge portions of the Low Countries have been prosperous and below sea level for centuries. It requires dykes, ditches, windmills and uninterrupted competent engineering organisations. Dutch history shows that’s adequate for keeping land 2m below sea level inhabited and rich. Any rise greater than that and they might have to break out technology more advanced than windmills. It’s a massive engineering challenge but unless Bangladesh is built on limestone so the bedrock is porous it’s an engineering challenge that WWII technology would have been sufficient to.

It's all but certainly too late for birthrate changes alone to reduce population within the anticipated timeframe.

That leaves emigration or mortality increases, barring further food miracles.