I had same problem with SIM548. My solution of this was set up address of DNS server (for example 208.67.222.222 - openDNS.org)
AT+CDNSCFG="208.67.222.222"
On SIM548 I must set up this every time after turn off this module, but on SIM300 I did it only once. Is it possible save this settings also on SIM548?