You do not need to forward the ports on both the router and the modem. First, you should ask your ISP to help you bridge the model so it no longer acts as a router. Then all router settings will be done at the linksys router. You can not have two devices on your network that both act as routers. This will prohibit you from accessing anything from outside of your network.
Also, I recommend that you get everything working with port forwarding first. Then after that is working, setup the DDNS. IF you try to do both at the same time, it is harder to troubleshoot.