It seems as though there is still an issue with the latest firmware on the TP-LINK W8951ND.

According to an Avast Network scan, this vulnerability still exists. The DNS addresses on my router have been modified.
I have been suffering from pop-up ads when clicking on known safe links. It can only be a matter of time before the wrong person gets tired of this and sues TP-Link for negligence.

If you have upgraded the latest firmware, you can also follow the guide to change some settings. It should be secure enough. No worries.
http://www.tp-link.com/en/faq-573.html

As you said, "I have been suffering from pop-up ads when clicking on known safe links.", I don't think it is caused by insecurity of your modem.

This router is an absolute nightmare to use.

-terrible wifi which drops connections and is not capable of more than 15mbps between lan computers.

-impossible trying to change settings within web interface. When changing DNS settings, you MUST change other settings on the page otherwise it does not recognise the changes, and does not save them. This is the same when changing IP address of the router.

- changing password to router is useless, the entire web interface can be accessed using "user", "password" . this is actually a joke, right?

- still insecure after latest firmware. The only option is to use telnet to disable all access to the router, then you must reset if you ever want to change one small setting, which leads to changing ALL settings again because there is no option to backup/restore settings.

- who knows how many more bugs and vulnerabilities are possible?

As many have said before, TP-LINK must be the worst junk out of any electronics, ever.
Their hardware can't even be built like this intentionally to allow their spies easy access into foreign networks, it's just too unreliable.

You have forgotten add "in my opinion".