Aside from being considered non-canon, both the movie and OVA are set on the Steins;Gate worldline, the worldline created when Okabe saved Kurisu at the end of the original series.
Worldlines are a central concept in Steins;Gate, and doubly important in how Steins;Gate 0 connects to the main series.
The main reason SERN was so important in the original Steins;Gate is because they eventually use the time machine they take from Okabe to develop their own time machines and take over the world.