Do you have select the wireless network every time in the wireless menu? I don't know why restarting the modem/router would make a difference. If you have a password on the network, make sure it's saved in the keychain.
It's been awhile since I've had to set up a network (mine connects every time).
When you start up, is the network listed in the menu (the one with radio waves on the upper right menu bar)? When you click on it, does it connect? If it's not listed, you shouldn't have to restart the router. Open wireless networking and have it search. I think there's an option to have it always search and connect to that network.
I have an Apple Airport router, so they work well with each other.
