This is the first time I've heard of a slow connection with only 1 port of the router. Usually it's either all ports are slow, or people are satisfied. The '825 has so many problems, though, that I shouldn't be surprised.
Just to be complete. If you can return it, do so. Get another brand. Several have recommended Netgear. I have a Trendnet TEW-639 that was 1/3 the price of an '825 (gigabit wired, wireless only 2.4 GHz band, no 5 GHz band) but it actually works as advertised. I found the ultra-short range of the 5 GHz band to make it unusable for my purposes.
Otherwise, what happens if you swap the connection from port 4 to another port on the '825? Is the connection still slow, or does it go to 1 Gbs? If it speeds up, I wouldn't wrestle with it. Just swap it to the other port and, if you need the 4th port, try connecting either the TV or disk player there and see if they work as you need. You could also buy a cheap 5 port gigabit switch and connect it to one of ports 1-3, and then connect both the TV and disk player to the switch. Leave port 4 empty.
Alternatively, you can get into changing IP settings on your computer. I'm not much help there. Things like packet size.
Realistically, this router is pretty much a dog. Dlink has for all intents abandoned it. I would make do with it for a while, and then replace it with one that works as advertised.