You need to be on PPPoE (Always ON) mode. In this you can connect without a dialer. Simply switching ON the modem will establish the connection to internet.
1. Configure your modem in PPPoE mode.
2. Enable DHCP in your modem.
3. In the TCP/IP properties of your connection (all machines), set " Obtain IP address automatically ".
If still facing problem,
4. Disable any encryption used for WiFi and check.
5. Make sure that the MAC filter in " wireless " page of modem is OFF.
Problem exists ?
6. Assign internal static IP to machines and check. Say 192.168.1.25, 192.168.1.30.
7. Mention modem - model and make.
8. Start > Run > cmd > ipconfig/all, post the screen shot (all machines).