I could be barking up the wrong tree, but everybody seems to be so fixated on settings and configuration etc so much as to not even consider it could be a faulty RJ45.
Try plugging your Xbox directly into the router see if it gets anything using the yellow cable, if it does. try the netduma with that yellow cable. If that works its the black one that's faulty.
If the Xbox gets nothing too, you have a faulty port on you spectrum router.
Sorry you gotta rule out hardware too, and RJ45's fail.
If you get something on the Xbox on the yellow cable to Spectrum router, but nothing on the netduma, then you're back in the court of these guys. but is definitely a consideration that is currently being overlooked.