I'm a total noob, so I may have some terminology mixed up and completely wrong ideas - but I'm trying!!
Double NAT is where you have 2 routers performing NAT... if you have 1 router behind a modem (or router in Modem only/bridged mode) there should not be any NATing on the modem.
You should be able to find any IP addresses assigned by DHCP on your Zyxel by plugging a computer into another LAN port on the Zyxel and accessing the GUI (ie type in the modem IP address in a browser). If your Zyxel is connected to your router via your routers WAN port (like it should be for PPPoE) I dont think it will be assigned an IP address by your Zyxel (it will be assigned by your ISP).
On the status page just click your router and click info.
On OpenWRT router I created an interface called MODEM with an IP address in the same subnet as the modem. The modem is in a different subnet to the router.
When I first set up the MODEM interface I set the gateway to the IP address of the modem - this fails, and is not the way to do it! When I subsequently leave the gateway blank the internet works fine and I can access the modem from my LAN.
In my setup the WAN port on my router has a VLAN tag (eth0.2) - but I believe you can achieve something very similar with virtual interfaces (ie eth0:2) which you should be able to setup on your Asus from the CLI (I did this before I got the OpenWRT router on a RT-AC52U - but I no longer have that config).
So, the router has an interface and IP address on both subnets and an interface and IP address on the Internet (PPPoE).
Modem IP: 192.168.1.1
Router IP: 192.168.0.1
Router MODEM interface static IP on eth0.2: 192.168.1.2
Router PPPoE interface on eth0.2
Interfaces PPPoE and MODEM are, of course, in my WAN firewall zone.
I can't find my original instructions for my Asus... but from
https://forum.kitz.co.uk/index.php/topic,14621.0.html# Create virtual interface on router assuming modem is on 192.168.1.1
ifconfig eth0:1 192.168.1.2 netmask 255.255.255.0
#IP Tables v1.3
iptables -t nat -I POSTROUTING -s ! $(nvram get lan_ipaddr) -d 192.168.1.0/24 -j SNAT --to 192.168.1.2
#or IP Tables v1.4
iptables -t nat -I POSTROUTING ! -s $(nvram get lan_ipaddr) -d 192.168.1.0/24 -j SNAT --to 192.168.1.2
There appears to be an alternative where you can do the VLAN tagging on the modem, but I have always done it on the router...
VMG3925-B10B getting stats over WAN cable.There are loads of alternatives available if you are not comfortable with any of the above...
Result of a Google search[Moderator edited to simplify the above two links.]