After lots of fiddling I've finally figured out how to get the Zyxel working with IP Passthrough. In other words, the Zyxel is my modem, and I'm using it with my own WiFi router downstream. Here are the steps I followed:
1. Go to Network Settings -> Broadband -> Broadband tab and edit Cellular WAN 1. Switch off NAT.
2. Go to Network Settings -> Broadband -> Cellular APN tab and edit the first one in the list. Turn APN Manual Mode on and then change the APN to three.co.uk and switch PDP Type to IPv4.
3. Go to the hamburger menu and Restart the router. Don't pull the plug, just restart it from the UI.
4. Go to Network Settings -> Broadband -> Cellular IP Passthrough tab and switch it on. At this point if you go back to the Home screen, you will most likely still see a local IP address in the Cellular Info, probably 192.168.1.2. It will probably also report that it has lost connection to the internet.
5. Again, go to the hamburger menu and Restart the router.
6. Once the router has restarted, look again at the Cellular Info and it should show your WAN IP address, something like 10.123.xx.xxx or similar. The Status should also say it is Up.
7. Plug in your own Wifi router to LAN 1, NOT LAN 2/WAN!!! Give it a minute. Your router should eventually get assigned the WAN IP and you're good to go.
Notes
- The internet icon on the top of the Zyxel should be blue in IP Passthrough mode. Don't assume it isn't working because it hasn't gone green yet.
- The LAN2/WAN descriptor is extremely confusing. This is if you want to provide an internet connection to the Zyxel via Ethernet cable on LAN 2. There is an option to switch this on in the broadband settings. You don't need it.
- You need to regularly restart the Zyxel for things to work properly. I noticed that some things would 'half-work' if you just apply the settings but don't restart. If something isn't working as expected, restart the Zyxel from the UI (don't pull the plug).
- I did all of this while connected to my downstream router's wifi. Even though it might not have an internet connection at various points, you should still be able to access the Zyxel admin panel if your router is connected via Ethernet on LAN 1.
- Don't mess around with DHCP settings. Leave them as they are.
- I've not tried turning off the Zyxel WLAN (Wifi networks) yet but I don't see why it would cause any issues.
Here are some screenshots of my settings:
