When a sensor network becomes disconnected, there are still many nodes that function correctly. To reconnect these lost nodes with the sink, we can deploy UAVs in-between to relay data. In contrast to previous works treating disconnected nodes as independent data sources, we consider a cooperative approach where sensors form subnets to facilitate data transmission. We formulate the problem to maximize the number of nodes that can find a route to the sink, given a limited number of relay UAVs. We prove that the problem is NP-hard. We then propose a two-step heuristic algorithm for the relay placement problem, show that the algorithm runs in polynomial time and analyze its approximation ratio. Simulations verify the effectiveness of the proposed approach.