Call them and ask?
From a thread on the OpenDNS forums (dated Feb. 2010):
i spoke with Sprint today.
here is what the guy i spoke with on the phone told me.
"yes, sprint is redirecting DNS on port 53" (obviously we, at opendns,know this)
"yes, even for static accounts"
Apparently there may be a firmware upgrade for your Sprint device that allows OpenDNS usage, according to a linked PDF from Sprint in that same message.
You need split horizon DNS service.
Your ISP is apparently one of those that uses internal domain names for things that are intended to be visible only to customers attached to its network. It directs you to the WWW page at, say, https://internal.example.net./login in order to log in, where the domain name internal.example.net. from that URL doesn't exist outwith the view of the DNS namespace that your ISP provides through its proxy DNS servers to customers. (example.net., here, is of course a domain name owned by your ISP.)
Obviously, OpenDNS — which, ironically, also provides customized versions of the DNS namespace — doesn't know anything about such arragements. Nor can you tell it.
So you need what is known as split-horizon DNS service. You need to ensure that all DNS lookups for example.net. and its subdomains — i.e. every domain name that the ISP uses for this internal, customers-only, stuff — are directed to your ISP's proxy DNS servers, and all DNS lookups for all other domain names are directed to the OpenDNS proxy DNS servers.
There's almost certainly no way that you can do this with a domestic-grade router. Such routers lack the capability. You won't be able to do this by adjusting the DNS clients on the individual workstations on your LAN, either. It's not a capability built into any but a very few DNS clients. (Pretty much only MacOS has this mechanism.)
What you have to do instead is run a forwarding proxy DNS server, somewhere on your LAN. You configure that server to perform conditional forwarding, splitting example.net. and its subdomains off from the rest of the DNS namespace; and you configure all of your workstations to use that DNS server for proxy DNS service. For best results, so that you don't need to manually look up the IP addresses of your your ISP's proxy DNS server, you enable that Automatic from ISP setting again, and configure the conditional forwarding for example.net. to use your router as an intermediate forwarding proxy.
If the necessity of having to have a single machine powered on if any other machines on the LAN are powered on gives you pause, you could even run individual forwarding proxy DNS servers on each workstation, rather than one central one serving your entire LAN. Running a DNS server on each workstation isn't exactly a novel thing in the world of Internet. People were doing it as a matter of course some thirty years ago. Most current operating systems actually come with DNS server softwares provided as standard. If you have BSDs, Linuxen, Macintoshes, Solarises, and so forth, setting up a forwarding proxy DNS server is a simple matter of installing djbdns, PowerDNS, BIND, or suchlike and configuring it with the appropriate conditional forwarding rules.
The non-Server editions of Microsoft Windows are the glaring exceptions. Even Windows Server has a DNS server as standard. So it's only really if you have an all-Windows-workstations network, without any Windows Server machines or non-Windows machines, that you will suffer from not having the software capability right there in the box.
There are other ways to address this, of course: scripts that do complex dances to reconfigure things temporarily and log-in, bodges using hosts files. But they all suffer from the same problem that you have now: lack of automation. With forwarding proxy DNS servers and split-horizon DNS service, your ISP can move its DNS servers and its internal HTTP servers around to different IP addresses without your needing to touch a thing. The forwarding proxy on the router obtains the new IP addresses for DNS service via DHCP automatically, and any new HTTP server IP addresses are simply looked up in the usual way. With hosts file bodges, in contrast, every time that the ISP changes these things you'll have to re-edit your hosts files to catch up. And your ISP, thinking that you're using DHCP for this stuff, almost certainly considers itself under no obligation to inform you in some other way that is has changed internal equipment around.
Further reading
Best Answer
Keep things simple.
If the only problem you have is the access to the ISP login page, just add the url and ip adress of this page in the HOSTS file to avoid a DNS request for this web page...
The syntax is {URL} {at least one space} {IPv4 Address}
Somethings like: MyISPLoginPage.com 12.123.123
BTW: if you want to discover the bests DNS servers for you, use this software: NameBench
http://code.google.com/p/namebench/
Hope this help. Let us know.
P.S. C:\Windows\System32\drivers\etc\HOSTS