Umm, maybe but not clearly enough as screenshots... Anyone else know your Wi-Fi password? If you reboot the router, can you login with admin? Next time, after reset it, during configuration, change...

The latest firmware fixed the problem?

27Mbps rate is not normal. http://www.tp-link.com/en/faq-409.html Have you reset them and repair them? Did you test the download speed bypassing powerline adapters?

A similar post here and I think the answer will make you disappointed. http://forum.tp-link.com/showthread.php?2238-Looking-for-the-AT-command-line&highlight=command

Have you tried to set APN and dail number manually on web interface of MR3420? If it won't work, you are suggested to report the problem here. http://ticket.tp-link.com/index.php?/Tickets/Submit

It is important to check WAN IP address on its web interface. How did you check or change DHCP settings on its web interface? It is not a necessary step.