Except Thanos still has it (the gauntlet might have burned, but he didn't leave all those gems littered around).
Expectations are that they use Quantum Realm shenanigans from Ant-man's portals to do so (apparently, Quantum Realm has weird time aspects). Probably tweaked by Tony (when he and Nebula have finished repairing the ship or whatever and reached Earth) and powered by Capt. Marvel.
I think the quantum realm stuff Lang was harvesting is what powers those portals mentioned in Ant man & Wasp, so instead of taking everyone there, it'll be the secret sauce to using Stark's Binarily Augmented Retro-Framing (B.A.R.F) to use technology for the time travel instead of just manifesting memories.