tp-link Archer C6 throttling speed?

Yes, it's the router.

That said, contacting support will probably accomplish next to nothing. I am not familiar with that specific router, but go through the settings to see if it has any throttling settings built-in and turned on (including energy-saver, QoS, duplex mode etc.). Also verify nothing else is using up bandwidth (a wifi router that's worth its weight in salt should list connected devices). Otherwise you're probably stuck with replace it or live with it. I cannot recommend TP-link for anything.

Edit: While switching cables couldn't hurt, it's unlikely to have a noticible impact unless one is faulty, old (cat4) or really long.

Edit 2: Also verify the modem is plugged into the uplink port (blue), not a downstream port (yellow).

Edit 3: If that doesn't work, I suggest you use it as an access point and get a real gigabit router without wifi. See "Access Point Mode" in the tp-link page you linked.