Why do TCP socket.connect calls fail ONLY on ios13 in 4g networks?

Question

we use a simple bit of socket open / connect / GET to download files for ios apps. This works just fine in almost all cases. EXCEPT on devices running ios13, on 4G networks.

The following combinations are all fine:

• ios12, wifi
• ios12, 3g
• ios12, 4g
• ios13, wifi
• ios13, 3g

But it simply fails when doing socket.connect() for ios13, 4g. The (simplified) code is:

#include <sys/socket.h>
#include <sys/ioctl.h>
#include <arpa/inet.h>
#include <netinet/tcp.h>

static void setsockaddr4( sockaddr_in *sa,String host,int port ){
    memset( sa,0,sizeof(*sa) );
    sa->sin_family=AF_INET;
    sa->sin_port=htons( port );
    
    if( host.Length() && host.Length()<1024 ){
        
        // Copy to char*
        char buf[1024];
        for( int i=0;i<host.Length();++i ) buf[i]=host[i];
        buf[host.Length()]=0;

        NSLog(@" BBSocket::setsockaddr4: hostBuff[%s]", buf);
        if( hostent *host=gethostbyname( buf ) )
        {
            if( char *hostip=inet_ntoa(*(in_addr *)*host->h_addr_list) ){
                sa->sin_addr.s_addr=inet_addr( hostip );
                NSLog(@" BBSocket::setsockaddr4: sin_addr [%s]", hostip);
            }
        }
    }
}

// ..
int _sock;
_sock=socket( AF_INET,SOCK_STREAM,IPPROTO_TCP ); 

sockaddr_in sa;
setsockaddr4( &sa,host,port );
printsockaddr4(&sa);
bool ret = connect( _sock,(sockaddr*)&sa,sizeof(sa) )>=0;
if(ret == false)
    {
        NSLog(@" BBSocket::Connect4. Error: %i", errno);
    }
    else
    {
        NSLog(@" BBSocket::Connect4. Success");
    }

For any of the correct networks I get through the connect call correctly, with a similar log to:

BBSocket::Open. proto: 1
BBSocket::Open. Socket open OK
BBSocket::setsockaddr4
BBSocket::setsockaddr4: hostBuff[s3.dualstack.eu-west-2.amazonaws.com]
BBSocket::setsockaddr4: sin_addr [52.95.148.124]
BBSocket::printsockaddr4: START -=================-
==- sin_family [2] :: AF_INET[2]  AF_INET6[30]
==- sin_port [80] :: netwOrder[20480]
==- sin_addr [52.95.148.124] :: netwOrder[2090098484]
BBSocket::Connect4. Success

However JUST for the specific 4g option mentioned above socket connect fails with an error 51:

BBSocket::Open. proto: 1
BBSocket::Open. Socket open END OK
BBSocket::setsockaddr4
BBSocket::setsockaddr4: hostBuff[s3.dualstack.eu-west-2.amazonaws.com]
BBSocket::setsockaddr4: sin_addr [52.95.149.16]
BBSocket::printsockaddr4: START -=================-
==- sin_family [2] :: AF_INET[2]  AF_INET6[30]
==- sin_port [80] :: netwOrder[20480]
==- sin_addr [52.95.149.16] :: netwOrder[278224692]
BBSocket::Connect4. Error: 51

The IP address is correct and all, just the connection fails with what is

error 51: ENETUNREACH:  The network cannot be reached.

Any insight on what might be causing this much appreciated, Thanks, Jaime