I get this too. Sometimes it passes. It's low priority at the moment as I lose all DNS after a while! I see you didn't get a response.... :-(

I had same when configuring ne new router up yesterday. Don't set under "LAN Settings" (unless you have local DNS servers). You want DNS requests to go out to the WAN through your LAN default gateway...

You should be able to upgrade or downgrade firmware. I'm considering this - I purchased new TD-W8968 and in 24 hours I have had to reboot 3 times to recover internet connectivity. Will post separate...