Hello Yanis,
Thank you for your query. I have just tested MQTT functionality on RUT950 FW 07.01.1 . Unfortunately, I was not able to reproduce the issue, WAN IP has been returned as intended.
Could you please specify what MQTT broker are you using? I have tested this with an MQTT broker called Flesbi.io, it is free: https://flespi.io
More information on MQTT configuration:
https://wiki.teltonika-networks.com/view/Monitoring_via_MQTT
Note: if you are using a different MQTT broker, please could provide more information what broker you are currently using and your MQTT configuration(from Server side, screenshot from WEBUI will do) and from the Broker Side. Alternatively, you could try using flespi.io.
Best Regards,
Dziugas