The collapse of White birthrates is equal to mass immigration in our peoples situation. There are many speculations as to why they collapsed such as decline in faith (countered by similar declines in religious nations like Poland) or poor economic conditions/lack of supports (countered by declines in White nations with strong security nets and supports for childbirth like Sweden). I'd say the main reasons for the decline are the rise of urbanisation, feminism and women entering the workforce.

A modern woman would rather have freedom and control over their life rather than devoting it all to raising a child. In a rural dominated nation with agriculture as the dominant sector, a child is useful to assist in farming, for urban and suburban, a child is really a burden from a material point of view. On top of these reasons their are natural factors that stifle fertility like rise in obesity, chemicals and women choosing to have children later in life.

How would a White nationalist movement aim to increase the White birthrate? Consider it from an activist pov (where the movement is now), a White nationalist party gaining power in a democratic system and in White nationalist dictatorship.