I had exactly this same problem trying to access a DLink DSL-320B-Z1 modem�s http admin interface through a Firebrick router. The modem does not have an option to define a gateway on its LAN-side ethernet interface, just as tdw42 said.

I solved it in the end by getting the router to rewrite packet headers, remapping addresses - not quite like the usual form of NAT we are familiar with. As tdw42 said the modem is only happy if it sees packets coming from src IP addresses in the subnet range that it can cope with and I suspect it can't successfully reply otherwise.

One solution that might work for you is to set the subnet mask in the modem to 0.0.0.0. I couldn't do so because the modem�s web ui whined. You could alternatively use netmask 255.255.0.0 or anything wide enough to encompass your existing real LAN subnet range within it too. (I couldn't do that because my main LAN uses routable global real IPv4 addresses, so no common high-order prefix bits.)

I am currently using a Billion BiPAC 7800N router.

I tried your suggested values but then found that I could neither access the User Interface, nor access the internet. Reset back to makers settings and OK.

What did not help was that there is so much scattered on different pages and I got the impression that I had lost access before I could get to the next page.

I thought that setting up a Cisco PAP2T for VoIP was a severe challenge, but the finer points of a Billion/DrayTek set-up may be an equal challenge

Cheers!

In reply to a post by ToxicSmoggie78:

Was there anything you needed to put into the Asus DSL-AC68U or was it just putting in the above on the Vigor 130?

The instructions above are for the settings on the DSL-AC68U - I didn't need to change anything on the Vigor 130.

Sure the web interface has no way to enter this information, but the command line does so you can issue something like the following on the Vigor via the command line

ip route add 192.168.1.0 255.255.255.0 192.168.2.100 static

and you are sorted.