It's canonically established that Star Trek: Enterprise is set within the Prime Universe rather than the Kelvin Timeline.
However, the show technically belongs in both. The Kelvin Timeline debuted in the 2009 Star Trek reboot movie directed by J.J. Abrams and starring Chris Pine as James T.
Based on Star Trek: The Original Series, the movie replaced every member of the cast with new actors, but it wasn't a traditional reboot.
Instead, every other Star Trek TV show and movie that preceded it remained part of the main canon, only in another timeline.
The return of Leonard Nimoy as Spock Prime alongside Zachary Quinto's new version of the legendary Vulcan confirmed this.
The Kelvin Timeline coexists with the Prime Universe.
Author's summary: Exploring Star Trek: Enterprise's timeline.