Sending A large binary payload via MQTT

Question

Hi, I'm using the serial_lte_modem sample application and I've modified the AT#XMQTTPUB command so that I can specify a slot number from which the device can grab a bank of binary data and send it up to an aws server. For testing, I just have a predefined bank of binary data and I use the slot number to determine how much of the buffer is sent;

AT#XMQTTPUB=<topic>,<msg>[,<qos>[,<retain>]]. So in my implementation, msg is required, it can be a string, then that message is published, it can be "" or 0, in which case the unit goes into datamode, or it can be a number, which refers to a slot. In the future, I'm going to be adding AT# functions to populate these slots with binary data, but for now, I have a fixed set of binary data and I use the slot number to indicate how many bytes of that data I will send (just for testing purposes);

if (strlen(pub_msg) == 0)
{
if ( binSlot == 0 )
{
/* Publish payload in data mode */
err = enter_datamode(mqtt_datamode_callback);
}
else
{
err = do_mqtt_publish(pub_bin, binSlot );
}
} else {
err = do_mqtt_publish(pub_msg, msg_sz);
}

OK, I've enabled CONFIG_NRF91_SOCKET_SEND_SPLIT_LARGE_BLOCKS so that I get around the 2k byte limit of TLS. but I'm still having problems going over 6047 bytes. Up to that point, the data goes over fine. Once I got 6048 and beyond I get;

AT#XMQTTPUB="<topic obscured>",6048,0

#XMQTTEVT: 1,-95

out the debug port i get; [00:00:48.454,010] <err> slm_mqtt: POLLNVAL, which I assume because it's still trying to do transmissions on a closed socket.

To facilitate the large transmission size I've added

#define MQTT_TX_BUFFER_LEN (64*1024)
#define MQTT_MESSAGE_BUFFER_LEN NET_IPV4_MTU

to create a special buffer for the MQTT Transmission;

static uint8_t tx_buffer[MQTT_TX_BUFFER_LEN];

I feel like there are other buffers I need to increase to avoid the EOPNOTSUPP, but I can't seem to find it.

I'm using SDK 1.8.0