cisco 871 no vpn tunnel

I have a site that the ISP nat’s one address to the address on my cisco 871 router. This was working on a watch guard SOHO before I changed out to a cisco 871 solution. Any ways I can not get the tunnel working. I can get to the external address and int my 871 but the tunnel will not connect. It also looks like my main router does not see this remote. Yes these are ipsec tunnels. Could it be the nating the isp is doing or a command I am missing on the on my wan interface or vlan interface? Thanks