Network expert with Router(NAT)

I am finding Nework expert on Android socket programing There are 3 devices Server(Android hotspot) Router(The ip is determined by DHCP of S(hotspot)) Client(The ip is determined by R(router)) When I send message from C to S, it is successfully transmitted. But when I send message from S to C, it is not transmitted If you can solve this problem, please message me Please include hotspot in your bid Thanks a lot