It’s quite unusual for ARC to show a “Ready” status but then fail specifically when you switch to cellular. Usually, if it works on Wi-Fi but not on LTE/5G, there is a hidden communication gap between your Nucleus and the outside world.
To help us get to the bottom of this, could you please share a screenshot of your Settings -> Roon ARC page from your Roon Server?
I’d like to verify that “Ready” message and see if there are any diagnostic codes hidden underneath it. Since you are using TM Malaysia with both an ISP router and a personal router, we are often dealing with a “Double NAT” situation where the connection looks good internally but is blocked at the second gateway.
On your mobile device, ensure that the Roon ARC app has permission to use Cellular Data in your phone’s system settings.
Once we have that screenshot, we can see exactly what the “handshake” is doing!