I have seen a preview to a movie or TV series on Netflix. The preview showed some super heroes (I believe) and they were walking around a city with large buildings. There were dinosaurs in the background and I believe they were just coming from or going to a portal. I have tried looking, but I just can't find out what it was. It is a newer show, not something old.
This sounds like a preview for the Season 3 premiere episode of DC’s Legends of Tomorrow. The episode is called "Aruba-con", and deals with the fall out from some timey-wimey mess made by the heroes, including dinosaurs wandering around modern-day USA and them subsequently being herded back through some portals:
According to @FuzzyBoots, this episode is indeed available on Netflix and is featured in the series preview on said platform.