The sunnah of using miswak will not be achieved by toothbrush. So it is mentioned in Fatwa Rahimia (vol. 4, p. 17, pub. Zakaria Book Depot. Deoband). The miswak should be of twig of a tree, if not, then rubbing the teeth with finger shall be in place of miswak. If you don’t have miswak and clean the teeth with brush and then clean them with your finger then you shall get the reward of miswak as well, in-shaAllah.
Allah knows Best!
Darul Uloom Deoband