Reset router to default settings and try another LAN port. Try to find an updated firmware for the router.
Try another router of better quality that can handle high speeds, if possible.
Test with another good quality ethernet cable, just in case.
Check device manager for device warnings and/ or conflicts. Update the network controller driver if needed.
Check for bitcoin virus or malware.
Just some personal thoughts.
What is the power supply unit exactly (PSU) ?