Use a different modem with SH2?

[Crimliar]

But home routers without issuing "routing rules" commands won't express LAN IP addresses on their WAN port - which is what the modem attaches to.

So set it up on a LAN port as a gateway?  Again with low/zero config consumer devices a really bad idea that causes all kinds of issues!

Solution:  The modem control WebUI sits outside of the router subnet (this gets complicated if you use a large subnet).  Internet data is sent via the gateway address, but control data uses that "reverse" channel to talk to the modem.

So on an Asus router it looks like this:

All internet data still goes the to ISP gateway, it's only that modem WebUI that goes to this address

[Crimliar]

I created that image for the response, and it's incorrect - the "IP Address is that of the WAN port and could be anything in the subnet - I used to use 192.168.2.10 not 192.168.2.1 which is the modem WebUI address and so would conflict!

[licquorice]

Don't know about Draytek, but the Huawei hg612 has 2 physical interfaces. One that you connect to the router WAN port and one that you can connect to the router LAN to provide access to the modem gui.

[Crimliar]

Yup the HG612 has a way more flexible UI, you can indeed set one of its ports to provide access to the modem UI on the router subnet.  But if you feel masochistic you can set it up the same way as the Draytek modems and just use a single cable using the same technique as the Draytek.  Out of the box, both modems are "plug and play", thanks to the "flexibility" of the HG612 it's quite easy for an inexperienced user to compromise their network if they start playing around with settings - as you do!

*The way spoofing the SNR on routers tends to work is that you sacrifice upload speed for download speed!

[TimCurtis]

@Casterina 

I'm using the SH2 as the front end for Digital Voice only with everything else turned off - firewall ,DHCP, wifi etc - and my Asus DSL AC68U doing it's normal job. Took a bit of effort to get it setup but had help/advice from @licquorice and now it works perfectly well.