I don't know what the TW tech will be doing, so I do not know if the TW tech is using the term bridging correctly. You can look at this
Bridging (networking) - Wikipedia, the free encyclopedia for more info. I'm not sure if it matters if TW bridges the modem to your router or not. I'll leave that for others in this thread to comment on.
Excluding bridging from the discussion - let me see if I can ramble on coherently about DNS settings:
The modem* gets:
an IP address from TW.
and DNS IP addresses from TW.
(A main DNS IP and one or more backup DNS IP addresses.)
For simplicity, I'll only mention one of your two routers
Your router gets:
an IP address from the modem
and one or more DNS IP addresses from the modem.
Each computer on your network gets:
an IP address from the router
and one or more DNS IP addresses from the router.
Normally, you can manually change the DNS IP addresses for the modem and/or the router and/or each computer.
The router has two main IP addresses:
The one that faces the internet (or that faces the modem)
and one that faces your network (maybe 192.168.0.1).
The router might (probably will?) hand out its 192.168.0.1 IP address as the DNS IP address to each computer on your network.
When a computer wants to surf to sevenforums.com
The computer asks the router,
"What is the IP address for sevenforums.com?"
The router asks the modem,
"What is the IP address for sevenforums.com?"
The modem asks TW's DNS server,
"What is the IP address for sevenforums.com?"
TW's DNS servers gives the IP address for sevenforums.com to the modem. The modem passes info to the router. The router passes that info to the computer. Wasn't that fun?
If you were allowed to change the DNS IP addresses on the modem, nothing much would change in that chain of events except where the IP address for sevenforums.com came from. If you did manage to point the modem to OpenDNS's DNS servers. Then the IP address for sevenforums.com would come from their DNS servers.
You can tell the router to skip asking the modem and go straight to OpenDNS for all DNS queries.
You can manually tell each computer to skip both the router and the modem and go straight to OpenDNS for all DNS queries.
Or - you can tell the router to automatically tell each computer to skip both the router and the modem and go straight to OpenDNS for all DNS queries.
*definitions are a bit of a problem here. Many of TW's "modems" are also routers and wireless access points. I don't know the make/model of the modem involved, but looking at
this page, I see lots of modem/router combos. Some of TW's simplest "modems" with only one Ethernet port also perform Network Address Translations, so I would call them a modem/router combo.
Caveats to the info above:
1) I think that is how stuff works - I could be wrong.
Corrections and/or criticisms are welcome.
2) For simplicity, several other configurations were not mentioned.
3) Even if the info above is correct, TW (and other ISPs) used to ignore your settings and force DNS traffic to go to its DNS servers. I do not think that any US based ISP still does that...
but you can see this post for links to discussions about that. I'm just not sure that I would call that DNS hijacking.