SIM card logging - What to check if SIM card is not logging?

If the middle light representing GSM network is blinking slowly (1 x every 1 second) then

1. check that antenna is properly connected
2. allow Roaming in Programming tables/GSM&SIM via EasyGate PC Manager. By default gateway allow a SIM to log only in home network. The gateway checks ID of the SIM and ID  of the network. As there are SIM providers using mobile network of other SIM providers, then these IDs are not same. If roaming is enabled then the SIM will log in such mobile network.
3. check which mobile network standards (GSM, UMTS) are supported by the SIM provider and check signal coverage. GSM models of gateways support only GSM networks. UMTS models of gateways support GSM and UMTS networks. You can find if you have GSM or UMTS model at the backside label the gateway.

You can find a list of mobile networks at Wikipedia article Mobile Network Code. This is quite reliable source how to find out which mobile network standard is supported by the SIM provider. You can also realise if the SIM provider uses a network of another SIM provider. If you want to be sure then check with your SIM provider. The list might be not up to date.