This is my case:
My local 3g frequencies aren't supported by the n900, so I'm forced to use edge.

My telco provides me with a cheap (1 ARS/day) unlimited data plan, that covers my daily messaging/mail/google needs.

The catch is that their connection sucks most of the time.
I've discovered that depending on the planets alignment and stock exchange, I receive a good or a bad edge connection... The bad ones have 20s+ pings with 30% drops, and disconnect after phone suspension. Bad connections are more frequent than good ones.

I can differentiate good connections from the bad ones by checking the ip that I'm provided. So I can blacklist bad ip ranges and reconnect until a good ip is assigned...
But what about asking for a good ip?

I come here to be pointed in the right direction.
correct me if I'm wrong.

APN's tell the network operator how to treat my connection... I tried changing it but I didn't obtain better results.

GPRS connects by ppp, and the IP is assigned by IPCP. During the Configure-Request I should be able to state which Ip address I desire (and pray for it to be provided).

So:
How can I do this in the n900? I know how to reconnect by dbus, but how do I really configure my ppp connection from the console?

If it doesn't work, is it possible to refresh the ip without dropping the the gprs connection, for the bruteforce approach I'm currently using?

Don't see the phone actually configuring ppp for the GPRS configuration.

IP addresses are assigned by the carrier and I don't think you can set it to whatever you like. I use services from two carriers and one gives you a NAT'd LAN address no matter how you connect to it and the other gives you either public standalone IP or NAT'd ones depending on the APN.