for the username/password, its case sensitive eg. Administrator different from administrator. access key just take from bottom of the modem, also case sensitive.

for the dns, follow below command and it will never show the router ip as 1st dns again. works on  mine TG784n V3. now i only get 2 dns that i set.

The default configuration uses DHCP to assign the router IP address (192.168.1.254) as the DNS resolver.
This method allows DHCP to assign different DNS settings to different DHCP pools.
In this case we use Google DNS for the LAN.
This method can also be used to have different DNS settings on the basic DMZ and advanced DMZ networks.

This will  not work on computers set to use a static IP address.

Telnet Commands
Commands

dhcp server pool list (view available DHCP pools, to help choice)
dhcp server config state=disabled (disable DHCP)
dhcp server pool config name=LAN_private state=disabled
dhcp server pool config name=LAN_private primdns=8.8.8.8 secdns=8.8.4.4 localdns=disabled  (set the LAN to use google DNS)
dhcp server pool config name=LAN_private state=enabled
dhcp server config state=enabled (enable DHCP)
saveall (make permanent)

hope this help...