Cannot run TCP server on iOS, cellular

I am trying to run a TCP server on iOS from Cellular. For purposes of asking this question, I wrote this very simplified version of a server and client in C using socket/bind/listen/accept/etc. This works fine when I run the server program from a server. When I run the server program from an iOS app via Cellular (no Wifi), the client never connects to the server. Here is my code:

Server:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <pthread.h>

#include "tcp_server_IPv4.h"

#define SZ_SOCKADDR_IN sizeof(struct sockaddr_in)

void pfail(char *w) {
  printf("pfail %s\n", w);
  perror(w);
  exit(1);
}

void *tcp_server_IPv4_thread_routine(void *arg) {

  int tcp4_sockfd = 0, tcp4_new_fd = 0;
  char wes[64];
  tcp4_sockfd = socket(AF_INET, SOCK_STREAM, 0);
  struct sockaddr_in their_addr = {0};

  struct sockaddr_in serv_addr = {0};
  serv_addr.sin_family = AF_INET;
  serv_addr.sin_addr.s_addr = INADDR_ANY;
  serv_addr.sin_port = htons(0);

  int br = bind(tcp4_sockfd, (struct sockaddr*)&serv_addr, SZ_SOCKADDR_IN);
  if (br ==  -1) {
      pfail("PROBLEM BINDING");
  }

  int lr = listen(tcp4_sockfd, 5);
  if (lr == -1) {
      pfail("PROBLEM LISTENING");
  }

  socklen_t gsn_len = sizeof(serv_addr);
  int gsn = getsockname(tcp4_sockfd, (struct sockaddr*)&serv_addr, &gsn_len);
  if (gsn == -1) pfail("getsockname");

  inet_ntop(AF_INET, &serv_addr, wes, sizeof(wes));
  printf("start listening on (%s) port (%d)...", wes, ntohs(serv_addr.sin_port));
  int tcp_running = 1;
  while (tcp_running) {  // main accept() loop
       socklen_t sin_size = sizeof their_addr;
       tcp4_new_fd = accept(tcp4_sockfd, (struct sockaddr *)&their_addr, &sin_size);
       if (tcp4_new_fd == -1) {
            perror("accept");
            continue;
       }

       inet_ntop(their_addr.sin_family, &their_addr.sin_addr, wes, sizeof wes);
       printf("server: got connection from %s\n", wes);

       if (!fork()) { // this is the child process
            close(tcp4_sockfd); // child doesn't need the listener
            if (send(tcp4_new_fd, "Hello, world!", 13, 0) == -1)
                 perror("send");
            close(tcp4_new_fd);
            exit(0);
       }
       close(tcp4_new_fd);  // parent doesn't need this
  }
  pthread_exit("exiting fire_up_tcp thread normally");
}


void start_tcp_server_IPv4() {
  pthread_t tcpt4;
  char *w4 = "TCP server IPv4 thread";
  int pr4 = pthread_create(&tcpt4, NULL, tcp_server_IPv4_thread_routine, (void *)w4);
  if (pr4) pfail("create thread for fire_up_tcp_server_v4");
}

Client:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#define PORT_NUMBER 57020
#define SERVER_ADDRESS  "70.208.71.248"
#define MAXDATASIZE 200 // max number of bytes we can get at once

int main(int argc, char *argv[])
{
  int sockfd, numbytes;
  char buf[MAXDATASIZE];
  char ip_str[INET6_ADDRSTRLEN];


  if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
       perror("client: socket");
       exit(1);
  }
  struct sockaddr_in sa_server = {0};
  sa_server.sin_family = AF_INET;
  inet_pton(AF_INET, SERVER_ADDRESS, &(sa_server.sin_addr));
  sa_server.sin_port = htons(PORT_NUMBER);


  inet_ntop(AF_INET, &sa_server.sin_addr, ip_str, sizeof ip_str);
  printf("Try connecting to (%s)(%d)\n", ip_str, ntohs(sa_server.sin_port));

  if (connect(sockfd, (struct sockaddr *)&sa_server, sizeof(struct sockaddr_in)) == -1) {
       fprintf(stderr, "Error on connect --> %s\n", strerror(errno));
       exit(EXIT_FAILURE);
  }

  puts("CLIENT::Connection successful.");

  char fr[128];
  sprintf(fr, "Some text here to send");
  if (send(sockfd, fr, sizeof fr, 0) < 0) {
       fprintf(stderr, "Error on send --> %s\n", strerror(errno));
       exit(EXIT_FAILURE);
  }

  if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {
       perror("recv");
       exit(1);
  }

  buf[numbytes] = '\0';

  printf("client: received '%s'\n",buf);
  close(sockfd);
  return 0;
}