The Pilgrims led 2-0 on the Jakemans Community Stadium 3G pitch before conceding six times to Solihull Moors. United added a third late on.
Rhys Smaller grabbed a brace for United, alongside a further goal from Jesse Ward.
Boston: Dan Martin, Joel Smith (Alex Van-Tam 66), Finley Mackey, Toby Cotton (Charles Doble 40), Dominic Chew (Leon Hagger 60), Kirill Snikeris, Will Pettitt, Zac Hardy, Josh Davenport, Jesse Ward (Kye Rook 63), Rhys Smaller (Louie Wain 63).
Referee: Ian Ruddock.