How to ping/telnet to the secondary WAN box?

[FabFlight]

I'm using an XT8 in dual WAN mode with failover.

From my LAN, I can easily ping or telnet to the modem on my primary WAN. How can I do the same to reach the modem on my secondary WAN, while it's in standby.

I haven't tried it yet, but I suspect I won't be able to reach my primary WAN when I'm in failover to the secondary. So, more generally, how can we connect to the modem (or ping/traceroute) to diagnose the on the standby WAN.

Thanks