3-way setup (LZW30-SN) with dumb switch


I am trying to replace a 3-way switch for my hallway lights. Box A (red) is upstairs where the light is, and Box B (white) is downstairs. I’ve read over the wiring diagrams more than a few times and finally believe that I have 3-wire in both boxes. So I am a little unsure how to wire up the Innovelli? and if I should put in in box A or box B? Help appreciated!!

Using a voltage meter,

Box A - Red wire is hot, black is not.

Box B - Red wire is hot, black is hot