Your problem is you’ve no idea which sketch your “used” emonTx V3 has, and even if it is still the factory default sketch, what has been changed by the previous owner. All you know is it appears to be functioning.
Not that you can do without being able to communicate with the emonTx - and this requires a programmer, or somebody with one who you can ask to help.
Go back to the emonPi and look at the emonhub log file - Setup → Emonhub → View Log. Do you see anything you do not recognise? The first line in the log when good data arrives is something like this (but more bytes of data / values for your emonTx), followed by more lines as the data is processed. It’s for an emonTH:
2023-09-22 16:19:26,184 DEBUG RFM2Pi 9696918 NEW FRAME : OK 23 172 0 0 0 224 2 26 0 4 0 0 0 (-50)
2023-09-22 16:19:26,186 DEBUG RFM2Pi 9696918 Timestamp : 1695395966.183707
2023-09-22 16:19:26,187 DEBUG RFM2Pi 9696918 From Node : 23
2023-09-22 16:19:26,188 DEBUG RFM2Pi 9696918 Values : [17.2, 0, 73.60000000000001, 2.6, 4]
2023-09-22 16:19:26,189 DEBUG RFM2Pi 9696918 RSSI : -50
2023-09-22 16:19:26,190 DEBUG RFM2Pi 9696918 Sent to channel(start)' : ToEmonCMS
2023-09-22 16:19:26,190 DEBUG RFM2Pi 9696918 Sent to channel(end)' : ToEmonCMS
2023-09-22 16:19:26,380 DEBUG MQTT Publishing: emon/emonth5/temperature 17.2
2023-09-22 16:19:26,383 DEBUG MQTT Publishing: emon/emonth5/external temperature 0
2023-09-22 16:19:26,385 DEBUG MQTT Publishing: emon/emonth5/humidity 73.60000000000001
2023-09-22 16:19:26,388 DEBUG MQTT Publishing: emon/emonth5/battery 2.6
2023-09-22 16:19:26,390 DEBUG MQTT Publishing: emon/emonth5/pulsecount 4
2023-09-22 16:19:26,392 DEBUG MQTT Publishing: emon/emonth5/rssi -50
2023-09-22 16:19:26,395 INFO MQTT Publishing 'node' formatted msg
2023-09-22 16:19:26,395 DEBUG MQTT Publishing: emonhub/rx/23/values 17.2,0,73.60000000000001,2.6,4,-50
'OK 23 ’ tells me the data checksum was good, and it came from Node 23.
You might want to download the log file and look at it with an ordinary text editor.
If there’s anything you don’t recognise, copy the whole line and the next few lines and paste into a post. This will only work of course if your emonTx is transmitting by radio. (It is possible the radio has been turned off if the first user was using Wi-Fi or a serial connection. Your emonTx does NOT have an ESP8266 attached?
Please put 3 ‘backticks’ (normally found at the top left of the keyboard) on a line of their own before the log text, and 3 more backticks also on a line of their own after the log text:
```
log
```
You can add a language identifier after the first 3 backticks: ```php
or even ```text
if you don’t want any language markup applied.