How to access modem through router?

I’m trying to gain access to my TP-Link TD8817 modem through my Asus RT-N66U router. I have my modem IP set to 192.168.0.254 and my router IP set to 192.168.1.1.

I believe I need to run the following scripts…

ifconfig eth0:0 192.168.0.253 netmask 255.255.255.0

and

iptables -t nat -I POSTROUTING -o eth0 -j MASQUERADE


Or add those scripts to startup files like this..

#!/bin/sh
ifconfig eth0:0 192.168.0.253 netmask 255.255.255.0

and

#!/bin/sh
iptables -t nat -I POSTROUTING -o eth0 -j MASQUERADE

That part I understand (I think/hope?) but I can’t find anywhere to add the startup scripts in the N66U’s GUI, and feel free to laugh but I have no idea how to create a file on the router from a shell/telnet etc. (I'm a complete Linux noob)

I'm using Merlin's firmware and I’ve enable and formatted the JFFS partition and I managed to gain access to it via telnet but that’s as far as I can get and I’m completely lost after that.

Could anyone give me an idiots guide on how to install these scripts so they run at startup?

Thanks in advance.