Nighthawk RAX200 - ethernet port becomes disabled randomly

> [...] any of the ethernet ports on the back of RAX200 become disabled,
> meaning I can't access the internet through the device on that port.
> [...]

   Does that mean that one of the ports fails at a time, unrelated to
which device is connected to it?  Other ports continue to work?

   Do wireless device connections also fail when this happens?  What all
fails when one failure is detected.

> [...] when I connect a device through that port once it has become
> disabled, my device can recognize, assign itself an IP address but
> cannot connect to the internet. [...]

   Does "assign itself an IP address" mean that your (unspecified) "a
device" got a self-assigned address in the "169.254.x.y" range?  That
would suggest that the device can't communicate with router's DHCP
server.

> How do I get ports on the back of the router to not randomly disable?

   Blame assignment is Job One.  Test/swap cables.  Move devices to
different ports.  Observe/record which devices fail when connected
where.

   I wouldn't expect a single port to fail to communicate on its own,
especially if its port-status LED is still good, and then recover with a
router restart.  Bad hardware is always a possibility, however.