AOE: Because Ymir no longer obeys the royal bloodline it's requirement is no longer necessary to use the Founder's power - Eren therefore doesn't need to maintain contact with Zeke in order to complete the rumbling. Before he leaves Paradis he leaves his Founding Titan with Zeke still inside it, then controls it remotely from the island using the Founder's power. By the time the alliance realise Eren isn't leading the rumbling in person it's too late - despite destroying the Founding titan's body the rumbling doesn't stop, and all they can do is watch as they're ignored completely by the wall titans, who walk around them toward the fort to complete the rumbling.