We cannot tell from the info provided..
Please tell us what modem you are talking about.. actual make and model.
Is it a pure modem or a router as well??
If it is cable modem, did you shut it off for at least 20min.. which is required to obtain the public IP after a change of routers??
If you have some other type of modem give us info.
What OS and airport utility version are you using to do the setup?
If a computer have you turned on ipv6 to local link??