If you're not able to find any operators manually it might be that your sim or operator is incompatible with the device itself. For that, I would need you to specify your device model number and tell me what operator are you trying to use.
Also, as you said, try other SIM cards from other operators and look if there's any progress in connecting.
Regarding the right settings, most of the time SIM connects with your modem and all settings are gathered automatically. If there is any specific information on what your operator uses to connect with the modem - you should find it on the website of your operator or you could just give them a call.