In order to have a successful communication between the host (for example ESP32-PoE-ISO) and the Olimexino-NANO-GSM  boards you need to connect them as follows:

Host board UEXT pins --- NANO-GSM UART connector pins (the one on the SIM card side)
       GND (2) --- GND (4)
       TX  (3) --- RX  (2)
       RX  (4) --- TX  (3)

Also NANO-GSM has to be powered up. You can do that with external power supply 3.8V-4.2V to UART connector pin 1 and GND to any of the pins. Since the one on UART connecter will be in use for the UART communication you can use the one on the USB connector pin 4 (the one on the side of SIM800 module).

After pluging a SIM card and powering up the Nano-GSM you have to press and hold the button for about a second to start the GSM module. You will see the green LED start flashing couple of times fast while initializing and once it is done it will blink once every ~3 seconds. When this is done the module is operating.

Load a SIM800 library (for example "GSMSim 2.0.2" by Erdem ARSLAN) and load any of the examples. Olimexino-NANO-GSM doesn't have a reset pin attached (notice that R3 resistor on the Reset pin is NA). Compile, upload to the host board and open terminal on the respective baudrate (in the examples of the mentioned library it is 115200).