Solid Red Light and Reset does not work

Good point; what are the LEDs on the Ethernet port indicating? Try a different Ethernet cable. Try a different port on the router.

Other possibilities are faulty power supply, or faulty OS, or faulty HDD.

Power supply can be replaced.
Data can be recovered from the HDD if the OS has become corrupt (and then the firmware can be re-installed).
Faulty HDD is pretty fatal; professional, low-level data recovery (expensive, not necessarily successful). Look to replace the HDD and recover data from your backup (or replace under RMA if within warranty).

For Gen1/v4 devices: look for ‘debricking/unbricking’ guides if gen2/v2.

Other possibles:

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