Configuration to access CenturyLink C3000Z DSL modem (bridge mode) GUI from LAN w/ ASUS RT-AC88U

I have a Zyxel C3000Z DSL modem/router from CenturyLink. I have, and prefer, my ASUS RT-AC88U router - so I've disabled the router in the modem by putting it into transparent bridge mode. Everything works well, except I cannot access the modem GUI from my LAN. I've seen several posts on how to do this, the two closest to helping included one referencing some setting supported in Tomato specifically for this case, and another requiring creating an additional address on the ASUS WAN interface...

Configuration to access CenturyLink C3000Z DSL modem (bridge mode) GUI from LAN w/ ASUS RT-AC88U