|The problem is if I connect my wireless routers by using the internet connection I have no internet or network connection. |
It sounds to me like you're just not configuring your router correctly or the modem may be looking for a specific MAC address. The TCP/IP settings you have applied to whatever NIC is presently connecting, and working, are the same settings you need to apply to the WAN (external) port of the SOHO Router. Also, find out the MAC address of what you've been connecting with and then change the external MAC of the router to that address.
Once you have that interface setup correctly and the router is able to ping external sites then your internal connections should work. It's worth noting that if you use the router and it's builtin NAT (network address translation) you do not need the extra ISP supplied IP addresses. You can simply use the private Class C addressing the LAN side of the router will want to use by default.
I wouldn't worry about DD-WRT. First, your router has to support it, and second, Tomato is a whole lot better. Especially if you get the "VPN" capable version of tomato.
As for adding another router, if you're going to wire it to the first one (I highly recommend doing it this way if at all possible), click on my name, above in my response, and read my "how-to" guide on adding a second router. As for using bridge mode on a router, read the router's manual on setting it up.