I have a Zyxel P-660HN-T1A modem, which connects to my ISP. I have an 8-port ethernet switch connected to it and other laptops connect via Wireless. As soon as upto 7 laptops connect to the modem, the Zyxel doesn't let any other device connect. If I try from my phone [HTC HD7, works fine otherwise], I get the error: Could not reach network! I am at a loss, what is the issue and how do I resolve it.
My ISP's zyxel had a 5 DHCP lease limit (non-expiring leases). We never have more than 5 devices around at any moment, but many visitors each day, so we had to reboot it every morning to allow new machines to connect. I reconfigured the zyxel to 50 DHCP leases max. No problems after that (I google for my ISP's password).
It could be a hard limitation of the firmware. Your best bet is:
1) Call your ISP and bite their head off
2) Turn the routing on your modem off, and get a proper router behind it.