NETGEAR Nighthawk routers Ethernet link is limited to 100 Mbps or disconnects

The checks in more detail

if the modem is fast and the router is slow, suspect the router's firmware, so update it and power cycle the modem and router. Check the device's own Ethernet adapter, since a 100 Mbps adapter caps the link regardless of the router. If wired speeds are good but wifi is slow, that is the wireless part. On multi-gig models the 10 Gbps or 2.5 Gbps ports only deliver those speeds with matching cable and devices. Check that no switch in the path is limiting speed, and that the port is not shared by many devices.

Blinking means traffic, solid means connected. Tag each cable at both ends with the router port it uses. A cable that is slow in one run is often fine in another, and cheap flat cables and old leftovers are the most common culprits. If you are wiring a house, test each run from end to end before the wall is closed, because a run that links at only 100 Mbps is costly to redo later.

Remember that a satellite connected by wire needs its own good cable to the router, and a run through a wall plate can add a poor joint that a direct patch cable would not. Games consoles and desktop PCs benefit most from a cable, so run one good cable to each rather than fixing wifi for them, and use the port LED color as a quick check that each link came up at gigabit. A cable that has been pinched behind furniture is a frequent hidden fault, so follow the whole run before replacing anything else.