Connecting to a device on the local network over TCP *sometimes* fails with "No Route To Host" socket error

I'm making an Android app (using Unity and C#) which needs to connect over TCP to a different app running on another device (using the .NET System.Net.Sockets.TcpClient and TcpListener classes). The way this works is the host app broadcasts a UDP packet giving info on its local IP (192.168.x.x) and its TCP listening port. Upon receiving the UDP packet, the Android app attempts to connect to the TCP endpoint given. Most of the time (~80%), this works perfectly and the two devices establish a valid TCP connection. Sometimes though, the Android app receives the UDP packet, tries to connect over TCP but a "No Route To Host" socket error shows up instead; even trying again upon receiving the next UDP packet fails.

I'm suspecting that this is to do with the router creating different subnets. I'm not very familiar with networking code, so I'm not sure how to forward the TCP request over to a different subnet of the local network. What's weird is that the UDP packet is always received no matter what; and most of the time, the TCP request will fail for 10 minutes straight then start working again like nothing happened.

public async Task<bool> ConnectToHost(IPEndPoint endpoint) {
    try {
        TcpClient client = new TcpClient();
        client.NoDelay = true;
        IPAddress ipv4 = endpoint.Address;
        Debug.Log("IPv4: " + ipv4);
        await client.ConnectAsync(ipv4, endpoint.Port);   // <--- this call throws the SocketException
        Debug.Log("Connected.");

        // ...

        return true;
    }catch(SocketException se) {
        Debug.LogError("[TCPClient] Socket Exception (" + se.ErrorCode + "), cannot connect to host: " + se.ToString(), this);
    }catch(Exception e) {
        Debug.LogError("[TCPClient] Error, cannot connect to host: " + e.ToString(), this);
    }
    return false;//Could not connect
}

The ConnectAsync() call fails on the client side, giving the following SocketException (error code 10065, WSAEHOSTUNREACH); on server side, no trace of the message is ever seen.

05-29 13:31:54.591: I/Unity(24587): IPv4: 192.168.1.21
05-29 13:31:54.591: I/Unity(24587):  
05-29 13:31:54.591: I/Unity(24587): (Filename: ./Runtime/Export/Debug/Debug.bindings.h Line: 48)
05-29 13:31:58.103: E/Unity(24587): [TCPClient] Socket Exception (10065), cannot connect to host: System.Net.Sockets.SocketException (0x80004005): No route to host
05-29 13:31:58.103: E/Unity(24587):   at System.Net.Sockets.SocketAsyncResult.CheckIfThrowDelayedException () [0x00014] in <9eab73f5583e4ab3921ff80e74ccdb29>:0 
05-29 13:31:58.103: E/Unity(24587):   at System.Net.Sockets.Socket.EndConnect (System.IAsyncResult asyncResult) [0x0002c] in <9eab73f5583e4ab3921ff80e74ccdb29>:0 
05-29 13:31:58.103: E/Unity(24587):   at System.Net.Sockets.TcpClient.EndConnect (System.IAsyncResult asyncResult) [0x0000c] in <9eab73f5583e4ab3921ff80e74ccdb29>:0 
05-29 13:31:58.103: E/Unity(24587):   at System.Threading.Tasks.TaskFactory`1[TResult].FromAsyncCoreLogic (System.IAsyncResult iar, System.Func`2[T,TResult] endFunction, System.Action`1[T] endAction, System.Threading.Tasks.Task`1[TResult] promise, System.Boolean requiresSynchronization) [0x00019] in <a6266edc72ee4a578659208aefcdd5e1>:0