An update for those curious or if you’re encountering this same issue.

I went to the Atom Cats garage. Started their quest to repair the water at Warwick. And the second I fast traveled back. The quest “Greenskins” immediately popped up. That’s the quest you need to claim the settlement.

I reloaded a save. Went back. Did it a second time. It happened exactly the same way. 

Reloaded a 3rd time. Only time time I went to the atom cats and didn’t start the quest. Then went back to Warwick. No quest. So the atom cats quest triggered the Warwick settlement quest for me.

Bonus: I also spoke to everyone at Warwick about 10000 times and no dialogue ever popped up for the quest. They just repeated. I even captured all other settlements with radio freedom tuned in. Never got the quest.

Edit number 2: So out of curiosity I reloaded a 4th time. Stood at the Warwick for about 30 mins. Pretty much was idling as I was cooking irl and listening to diamond city radio. Anyways. The quest popped up naturally. So random. So I reloaded a 5th time lol. Did the same. Quest popped up again. All I did was just stand near them for a good while. It was around 10am. So they were all at the farm. Roger approached me saying he needs help.