dns lookup only works with DHCP and not with static setup (EtherCard library)
DHCP does more than just give you your IP address: it also gives you the Net Mask, the Gateway IP address, and the DNS Server IP address. If you don't call dhcpSetup(), you'll have to do it yourself.

According to the EtherCard documentation, you can do that as follows:

const static uint8_t ip[] = {192,168,0,100}; // Example
const static uint8_t gw[] = {192,168,0,254}; // Example
const static uint8_t dns[] = {192,168,0,1};  // Example

...

if (!ether.staticSetup(ip, gw, dns)
{
    // handle failure to configure static IP address
    // (current implementation always returns true!)
}