How to access 2nd router (ASUS RT-AC68U) login page while on main router (Fios G1100) WiFi network?

Hello!

I'm wondering if this is possible, please let me know.

My setup is currently:
Summary: ASUS router is behind the Fios Router

1.Verizon Fios Quantum Gateway G1100
(connection shared via Ethernet cord Fios LAN to ASUS WAN)
2. ASUS RT-AC68U running asuswrt-merlin 384.7.2

Issue:

I noticed that while on my ASUS WiFi/Router, I CAN ACCESS the Fios Router login page and the ASUS Router login page both with no issue.

But when...

How to access 2nd router (ASUS RT-AC68U) login page while on main router (Fios G1100) WiFi network?