NRF Uart With sim868
You can't connect the TX lines from two modules together. If you want to use your "TTL module" to monitor the communication then you can only connect the RX line. So, you can only monitor one direction of data transmission at a time.

Of course, you must also have compatible voltage levels for the serial interfaces in all three devices. You haven't provided enough information to answer that, so I'll just have to trust that you have verified this.