Odd issue with Internet connection

Hi,

Posting this here, as I think it's a router problem.
A frined of mine connects to the Internet (Ethernet) via a Pirelli router (don't ask for more info, no idea).
He mostly uses OpenDNS and Comodo Secure DNS, but due to some restrictions to his router by his ISP, he sets the DNS Servers through Windows (XP SP3).

He's having this issue:
His Internet connection goes down due to his ISP having problems. When the connection gets restored by his ISP, he can't go out to the Internet unless he does one of the following:
1. Change his settings to "Obtain DNS server address automatically" - as long as he keeps it this way, everything works fine, but if he changes his settings to "Use the following DNS server addresses" and enters the IPs of any other DNS Servers, he's back to zero again.
2. Reboots the router (it has a switch at the back) - doing this, makes the router work with the alternative DNS Servers and (of course) assigns him a new IP.

He's almost sure that this was not happening from the beginning.
I can't figure out what sort of problem this could be (there's no logic in the router not recognizing alternative DNS Servers), but there's definetely something weird going on.

He's got this router for about 1.5 year now - it was provided by his ISP, and before someone says "replace it with a better one", he can't because his ISP also provides him with VOIP services which only work with their router (I think it's a firmware thing).

So, is this a router gone bad? Could it be something else, like a phone line issue?

Any help highly appreciated,
Flying Dutchman