The USS Enterprise didn't join the first battle shown in Star Trek: Discovery.
Starships Clarke, Shran, T'Plana Hath, Kerala, Sue, Ride, Earhart, Dana, Edison, and Yeager have dropped out of warp.
Where is the USS Enterprise (any version) during the events of Star Trek: Discovery?
In Star Trek: Discovery S02E01 ("Brother") we learn that the USS Enterprise, under command of Captain Christopher Pike, was on its 5-year mission when the war started (which covers Season 1 of DIS).
It was not supposed to participate unless absolutely necessary. Even then, they were far away from the war, so it would still take time to join the war effort.
PIKE: Missing the war while the Enterprise was on its five-year mission, that, uh took a toll on my crew, a toll on Spock. On me, too.
BURNHAM: Even if they had called you back, you were so far away, you wouldn't have made it in time. Starfleet ordered you to stay away for a reason. Enterprise was an instrument of last resort.
The Gamma Hydra sector was visited by the Enterprise in 2267 in 'The Deadly Years' (TOS) and is near the Neutral Zone established after the Earth-Romulan War (TNG 'The Defector').
In the map of the explored galaxy commonly shown in various episodes (below), the Neutral Zone is to the far left of the image. Now, this map is first seen in Star Trek VI (2293) (Source), so the Enterprise should (barring any strange/unreported instances) have been within the confines of this map. Looking at the three systems it has been, we see Rigel is 3 cubes from the Neutral Zone in a straight line. Since in 'Errand of Mercy' (TOS) was set in Organia and the territory was disputed between the Federation and Klingons, the border was probably closer to the bottom right edge of the map, which would be even further.
We also know that a Constitution class vessel would take two weeks at maximum cruising speed to reach Rigel from Talos (Source), which is travelling five 'squares' on the below map. Add another three 'squares' to travel in a straight line from Rigel to Organia and this increases to just under three weeks.
Another consideration is that the Enterprise's mission was of exploration, so they probably wouldn't have travelled around space at maximum speed, but cruise slowly to explore the galaxy.
So, although Talos isn't the farthest point from where I suspect Gamma Hydra is, it just about is. Assuming the Enterprise was at least an average ship in terms of her engines (sorry Scotty), it would take her just under three days to traverse a 'square' on the map. Therefore, all the ships called to the Shenzhou's position must have been inside the 'square' of the Gamma Hydra sector. Since the Enterprise didn't arrive, it mustn't have been anywhere reasonably close.
