I'm not sure why D-Link's website doesn't show this product, but it may be because it's either intended for their business partners (i.e., available to and distributed only by ISPs), and/or perhaps intended for a foreign market and would only appear if connected to that particular part of their website.
If it’s the former, it may very well be impossible to update the firmware, per agreement between D-Link and its partners (i.e., alternatives may not be available). Even if you could obtain it somehow, the router may be LOCKED DOWN to prevent tampering. Or worse, permanently break your connectivity due to the lack of proper configuration.
In most cases, it shouldn’t be necessary anyway. You can always patch another router behind the existing modem+router and thus manage your own network as you see fit. Of course, you’d be double NAT’d and have to manage two firewall for remote access purposes. But that can usually be mitigated. In the case of the former, by placing the modem+router in bridge mode, and the case of the latter, by giving your own router’s WAN a static IP in the same network as the modem+router and placing that in the DMZ of the modem+router.
So at the end of the day, it’s probably more reasonable to assume you can’t change the modem+router and instead incorporate your own router using a proper configuration.
Now that you mention it, I guess it is a case of a locked router, it has the ISP's logo embedded into the shell.
I get tiny breaks in the connection that always cause me to disconnect from certain places or stop downloads. I believe(d) it is the cause of the firmware being buggy and causing those problems. that is why I wanted to replace it.
I guess I could just ask my ISP to give me a DSL modem instead and I will use my older router, or if you can point out on a good combo router I just might get that instead and not bother with my ISP.
I would definitely go to the ISP first since it's their equipment and it should be working normally (that’s what you paid for, so demand it). And it might be some other problem on their end too, so switching in your own modem+router might not solve the problem anyway.
If you do pursue your own equipment, I do NOT recommend getting a combo device, but separate devices. Those combo devices are nice for the ISP since it’s indeed easier to lock down, easier to manage, even cheaper, but as an end-user, it also limits your choices, and all because of the stupid modem, which does next to nothing anyway. Better to have a working, solid modem, then choose whatever router you want. Also, most of these combo devices don’t expose a WAN port, but instead the connection between the modem and router is *internal*. So if you decide to switch to cable or fiber (FIOS) down the road, where you need an exposed WAN port, you’re out buying new equipment.