You are not using DSL but ADSL so your uploads won't affect you like as if you were using dial up or something. So change the upload slots to at least one or I'd recommend 2-3. This can help you find more sources when searching by connecting thru more people.

I've heard that D-Link is better used in port forward mode (like I have my router configured). You can check here but I don't see your model mentioned http://www.portforward.com/routers.htm Otherwise check D-Links website for support. Look for it under the UPnP section here Connection problems? Check here first! (click on link)

Your device might not have an internal NAT firewall. But that I don't know. You'd need to speak to support via your isp or D-Link to find out.

What bothers me is one moment your speeds were fine. Next month they were suddenly miserable. Sounds like the ISP has turned a switch on. Such as a p2p filter. They will "never" tell you this because most employees don't know & the company probably also have a policy of not telling customers. That's how it is in the USA & many parts of Europe.

Have you heard of iinet in NZ?