Get another second hand box from vy-vy Ecotec. They’re the same trans.
If you’re having troubles in 2nd, you’ll generally have the same issues in 4th because it’s related to the 2/4 apply piston and how it applies the band. Band could be worn, piston may be damaged, housing may be cracked, accumulator may be damaged or leaking, vb gasket may be damaged.
Most likely just a worn band if it slips when going into second though. Whichever way it’s failed, most cost effective option is to replace with a good second hand transmission. Usually $100-300 with warranty depending where you go. Plus fitment of course.