Hard reset the M7350

The connection changed from PPPoE to others? Do you confirm with your ISP the connection type and what to put in the tplink router?

You can refer to http://www.tp-link.com/faq-735.html

Does the WAN IP address change? Try a different PC to access your router. If the internet works, there should not be a problem, cause the IP address is correct and you can ping the router's IP. Maybe...

If none of those options is ticked, it's disabled, not one can remotely access it.

That should not be a problem. Did you try a different USB port? Does your HDD need an additional power supply? Or does the format change? What's the capacity? And better confirm the model number,...