apotts wrote: You can do this without hardware.

Install Asus Merlin firmware on your router. It's just tweaked and improved Asus firmware - and do a factory reset and manually re-enter settings after the upgrade.
Use this guide to set ip addresses: https://github.com/RMerl/asuswrt-merlin/wiki/Access-modem-Web-UI-on-WAN-port-(no-script)

It then just works. I use the same addresses on my network as shown in that wiki. So my Asus is on 192.168.1.1, and yet when I go to 192.168.0.1 my Vigor 130 page appears like magic.

You don't need to do any of that. It works with Asus stock firmware as well as Merlin. Simply connect the devices and using default settings 192.168.2.1 produces the DrayTek GUI .

I've got this sorted now and it works a treat, and for those who want to know how to do it, here is how I got it working (at this point I thought I'd mention that I've changed my network setup, still using the Vigor 130, but now with a TP-Link Archer C3200 router. Not sure if this will work with other routers, but I can't see why not if you are able to set up a secondary connection on your router):

- Lets assume your modem is on IP address 192.168.2.1 and the router is on 192.168.1.1.
- Login to the routers GUI / admin page
- Go to Advanced -> Network -> Internet -> Advanced
- Specify a secondary connection linked to modem by entering info as follows:
1. Connection type: Static IP
2. IP address: 192.168.2.3
3. Subnet Mask: 255.255.255.0
- Leave all other settings alone and click on 'Save'

After you log out of the routers GUI / admin page, you should be able to access your modems GUI / admin page through the router using the modems IP address.

Hope that is of use to someone!