Read variable value from mqtt

Hello everybody.
I have a sonoff th16 with tasmota firmware. I followed the thermostat example in the “rule cookbook” guide. In this example the “mem2” variable is configured to set the maximum temperature. The instructions read:
“Mem2 25 <- setpoint Temp upper limit - View or set by MQTT cmnd / mqttTopic / mem2”.
I found a way to change the value through a script, but I can’t figure out what I have to do to read it.
I tried to configure a sensor like this:

 - platform: mqtt
    name: "Set point temperatura massima acquario"
    state_topic: "cmnd/tasmota_th16_riscalda_acquario/mem2"  

but the state is not recognized.

This is what I get from the tasmota console when I type “mem2”

13:47:13.747 CMD: mem2
13:47:13.755 MQT: stat/tasmota_th16_riscalda_acquario/RESULT = {"Mem2":"28"}

Can anyone help me? Thank you