Yes, the LAN address has to be static and properly forwarded or else it won't work perpetually as it will often get a new address.

People will often find, they run the setup, it WORKS and it continues to work for a FEW DAYS, but then stops. This is because, normally default LAN DHCP IP address reservation is from 24-72 hours, so the slingbox MAC will get the same IP address for that period of time. Then, it can change and when it does, the forward is essentially setup wrong since it is targeting what is most likely an out of date INTERNAL LAN DHCP IP address

DynDNS is great, but totally unnecessary for the slingbox, as the slingbox is calling out a few times a minute as it is so the slingbox server knows your external IP address sometimes even before DynDNS does. or no-ip.com, or whatever one uses. :-)

I use the slingbox server lookup to tell what my external addresses are for all my slingboxes worldwide. :-)