Who is the ISP and how would they stop you from getting the modem model number? Doesn't that just consist of you walking around your house and physically finding it and reading the model number off a sticker?
Also, you said you plug ethernet directly into the wall. No ISP runs ethernet in the streets, so you must have a device somewhere in your house/dwelling that turns ethernet into whatever your ISP uses.
As for carrier grade NAT, if they do use it, it could be what is stopping you from getting online. A router has NAT as part of it, so if your ISP also uses NAT, there can be redundancy conflict. It sounds like the router needs to be put in AP mode but not sure until you can tell us more about how your internet works.
What settings did you put into the Xbox? I wonder if they are conflicting with the router.
I didn't put the public IP settings into it, actually. I was supposed to configure them directly into the router, so I put it on hold.
Here's how it works: It's an apartment complex, one among many using this specific ISP. In my 19 years living here I've never seen a modem. There are two ethernet jacks in the wall in each apartment. Either you connect your computer directly as I am now or you first connect your router to it and then your computer to your router, either through LAN or wireless.
It shouldn't be a carrier NAT problem, really. My mom lives in the same apartment complex with the exact same ISP and service just across so she is affected just like me, unless the DIR-615 is really that different from her DIR-635? I can almost guarantee she never configured her router as an AP, although it is worth trying of course if you feel like it could help.
Doesn't it say anything that on the router page it says the DHCP client is disconnected and that it says that the internet is offline? Could it help by taking some screenshots of the relevant router pages?
Funnily enough, since changing security from AES to TKIP, instead of connecting to the WiFi and then getting a ''No Internet'' warning, it now simply says ''Unable to connect to this network''. Not sure if that's a sign of something.