Signs point to Ant-Man & The Wasp taking place sometime leading up to and/or during Infinity War
First, the most recent trailer starts off with Scott and Hope discussing the events of Civil War.
Additionally, Marvel's official synopsis specifically describes it as taking place in the aftermath of Civil War.
From the Marvel Cinematic Universe comes a new chapter featuring heroes with the astonishing ability to shrink: “Ant-Man and The Wasp.” In the aftermath of “Captain America: Civil War,” Scott Lang (Paul Rudd) grapples with the consequences of his choices as both a Super Hero and a father. As he struggles to rebalance his home life with his responsibilities as Ant-Man, he’s confronted by Hope van Dyne (Evangeline Lilly) and Dr. Hank Pym (Michael Douglas) with an urgent new mission. Scott must once again put on the suit and learn to fight alongside The Wasp as the team works together to uncover secrets from their past.
If the movie were to take place post-Infinity War, the official synopsis, at the very least would likely read differently.
In an interview with Seth Meyers, Paul Rudd revealed that the movie takes place "kind of concurrently" with Infinity War
“[Ant-Man and The Wasp] happens kind of concurrently and it’s very important because it doesn’t exist in a vacuum. You have to stay through to the end because once you see this thing through those post-credit scenes — there’s some mind-blowing stuff in there that kind of ties into Infinity War and it’s uh — I think I messed up. I think I messed up, yeah.”