Obviously since you can connect both localy and over the WAN, your basic setup is correct.  The problem at home as got to be an IP/routing one.  Your Zyxel LAN and WAN conenction work since this is where both your wireless LAN Sling conenction work and your various WAN accesses work (other than your Son's).  The problem at home has to be with your Desktop connection.  Although I assume you did double/triple check that you entered the Finder ID correctly in the Desktop Slingplayer.

I am a little confused though on exactly how you home is set up.  Is your Zyxel that the Slingbox is plugged into a wireless router and the same one your home laptop connects to?  When you say internet connection in plugged into the Zyxel, do you mean your VDSL line (your Zyxel is alo a DSL modem) or do you mean an ethernet port out of your intenet DSL modem plugs into your Zyxel.

Your desktop is not connected to your router, but is directly connected to your internet modem.  Is this the same internet modem that the Zyxel is connected to or do you have two diffferent internet connection (VDSL and something else) or two different DSL modems (one would be the Zyxel above)connected to the same VDSL?.

What about public IP addresses?  How many do you have and what device has it/them?

Again it sounds like your problems are network ones, not Sling ones really.  So the only way to help is to understand in detail your network setup.

OK first I am not  a Sling Media employee and I dont think your Finder ID would help anyway since we know it works over the WAN.

I understand your set up for the most part, it is similar to mine.  I am going to make the assumption that you are using double NAT.  That is that your DLINK router is using a private IP address range on the LAN side.  Since your Desktop and x-550 (wireless router) both are conencted to the DLINK (if I understand you description correctly) they should be on the same sub-net (same address range).  The x-550 is likely also set up to use NAT and has a different private address range it is using on its LAN side (hence your double NAT situation).  Since the Slingbox is connected to the x-550 it is on a different sub-net then your desktop.

My understanding of how the Slingplayer works it it first looks for a Slingbox on the same LAN as the player being used.  If it does not find one it uses the Finder ID to get the PUBLIC address of your outer most router.  This is the first router configured to portforward all sling traffic towards the slingbox.  It then sends the "are you there" (or something like that) over the designated port (default 5001)  to your PUBLIC IP address.  Your router (DLINK) knows to forward that to the slingbox (or actually in your case to the x-550 which in turns sends it to the slingbox).  Now the slingbox replies on the same port back to your PUBLIC IP address.  Now that router gets confused.  On one hand this is a reply from another address on the other hand it is supposed to send all traffic on that port to the slingbox.  I think you can see the issue.  I suspect it is a infinite loop kind of thing.

One thing I would try is on your desktop, go into the Slingplayer and then slingbox properties (edit when slingbox directory window pops up).  Under direct connection click on "For remote connection I prefer to use...IP adress...",  enter in the slingbox's actual private IP address.  I think using a direct internal IP may bypass the DLINK's port forwarding since the traffic should never hit teh WAN side.  I THINK it should pass through the x-5500 ok also, but maybe not.  I am not that much of an expert to be sure.  the port forwarding on the x-550 might still cause an issue.

If that does not work the only other thing I can think of is reconfigure your home network to all be on the same subnet.  That would mean disabling NAT on the x-550 and using it as a regular router.  While not exactly hard to do, it can be fairly involved and requires a fair understanding of IP addressing and sub-nets as well as router setup, DHCP passthrough, or static IP addressing etc depending on exactly how your routers work.  Its would be too much to try to describe here.  Between your x-550 deocumentation and/or some research on the internet you might be able to set it up.

Or conenct the slingbox directly to the DLINK.  Get ethernet switch and connect it to the terminal in your son's room.  Connect both the Slingbox and the x-550 to the switch.  This would put the slingbox on the same subnet as the desktop.  You would need to change the Slingbox network settings as its IP address would change, and you would need to change the DLINK portforwarding to point to teh new address and delete the x-550 portforwarding.  You might (porbably will) have a problem connecting using your wireless home laptop now.  If so, go to the "For remote connection I prefer to use...IP adress...",  on the laptop and enter in the slingbox's new private IP address.  That should work as the notebok traffic will only go through the x-550 which no longer has any port forwarding conflicts.