When setting your TP Link TD-W8968 to use 11n-only (rather than mixed mode), you get:
11n-only is unsupported with TKIP encryption enabled
This is, as the message suggests, because 11n doesn’t support TKIP on your router.
To fix this, navigate to Wireless -> Wireless Security and change the encryption to AES. You should now be able to enable 11n, instead of mixed mode without the router complaining that 11n-only doesn’t support TKIP encryption.