That makes sense why it would never fail the ARP test as your ADSL Modem/Router is still online. You should be able to achieve the desired result by putting the ADSL router into bridge mode and connecting the MX to it. You may have to look at the specific way to configure it on your router but the end result should be that the MX receives the public IP address and acts as your gateway router. Therefore when there's a failure anywhere upstream of the MX (whether it be the cable or the ADSL router in bridge mode), it should fail the ARP test and perform a failover to the secondary uplink.
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