NETGEAR Nighthawk RAX120 router internet or Wi-Fi is slow

The checks in more detail

so check QoS in Advanced, Setup, QoS Setup and make sure it is really off; QoS is designed for slower lines and caps a fast one. For a fade, a thread on bufferbloat explains that when the router is faster than the cable modem the buffers fill and speeds drop until a reboot clears them, so a router that is great after a restart and slow a day later has either this or a heat problem, and the community often links the fade to heat, so check the temperature as the overheating page describes.

For Wi-Fi speeds, place the router in the open and central, off the floor, and keep it away from other electronics and thick walls. Check the band the device uses: 5 GHz is fast but short range, and 2.4 GHz reaches farther and is slow. If the router uses one name for all bands, a device may hold a slow band, so give each band its own name and join the 5 GHz one. Use a 80 MHz channel width on 5 GHz and pick a clear channel in the router's Wireless settings.

Make sure the device's Wi-Fi adapter supports Wi-Fi 6, since an older adapter limits the speed. Update the router's firmware, reading the release notes and the community posts for a release that helped. Check for a link that has fallen to 100 Mbps, with a bad cable. Disable features such as Armor or Dynamic QoS for a test. If speeds stay far below the plan with QoS off, a wired test confirming the router and a clean channel, contact NETGEAR with your test results.