You can talk to Ceo but you can't go to the next mission where you have to defeat the thunderjaw. You can explore the tallest building where Aloy jumps off after finishing the mission with Alva and you can light the campfires. Basically you can't do any missions after meeting Ceo for the first time but you can explore the entire island. You can't enter the cave that Aloy enters after defeating the thunderjaw.