Since my crystal ball is currently in the workshop and you continue to skimp on important details, it’s not easy to help you.
If the Ubuntu installation is now running, like you wrote. Do you have ensured that you have updated all packages with sudo apt-get update && sudo apt-get upgrade -y?
Normally this is part of the argon1.sh script. This script additional provides a systemctl daemon ‘argononed.service’ for the fan control. Are you sure, that you have executed this script at your Ubuntu installation?
If you have disconnected the fan physically and connected the fan directly to the RPI5 pcb, the fan control only works if the kernel supports that. Additionally you must than configure a dtoverlay with the thresholds right and/or additional self made script to control the fan on that way: Argon V3 fan speed issues - #10 by HarryH