Are you connecting via WIFI? if so, be sure your wifi ssid is unique. On your device, delete (forget) the wifi network and reestablish the network.

If you can sign on to the router, you can see the MAC address in the advance settings.

Agree with Gray-Blue7 a TP-Link AC router with Gigabit ethernet ports is a good choice. Most AC routers will support IPv6 which you want. I have the Archer C8 connected to a Motorola MB8600 with...

If it was me, I would exchange it for a new one.

Do you mean you cannot log on to the router or to a specific site such as TPLINK.Com. If it is the router, I have had issues signing on time to time on my Archer C8. Sometimes http://tplinklogin.net/...

By not appear, do you not see it on your device? Be sure you have the "Enable SSID Broadcast" set on for the 5g. Also check your mode (11A/N/AC mixed).