Firmware version of your WA901ND is not the latest one. Upgrade them first. Both repeaters are receiving Wi-Fi siganl from the access point or one repeater is receiving Wi-Fi signal from another...

I'm afraid not. To upgrade the firmware, we have to use web brower to login web interface of the router.

PPTP VPN connection? TD-W8951NB has no such a feature and it supports VPN Pass-Through only. http://www.tp-link.com/en/faq-558.html

Media sharing? http://www.tp-link.com/en/faq-731.html

Maybe you can try to upgrade firmware of another MR3420 to be latest version as well to see if you can reproduce the problem. If it does, report the problem here....

What's the DDNS service provider? Did you press Save button after you finish DDNS configuration?