Can you login web interface of D7 on your Samsung phone? As the router is working with all other wireless clients, try to contact Samsung for some advice. If you don't mind, disable Wi-Fi security on...

Read the post here. It may help. http://forum.tp-link.com/showthread.php?78519-Random-fault-T4U-AC1200&p=152918&viewfull=1#post152918

Is it using smart IP or static IP? Have a check on its web interface, Network---LAN.

As I know, C9 is not designed as a multi-function router. But some other of their products have different modes.

Try to configure bandwidth control. http://www.tp-link.com/en/faq-557.html

What's the model number pls? Upgrade the latest firmware, reset and reconfigure it to see if the problem continues.