Hello,
Standard SIM should be used unless you have the adapter for your smaller formar SIM's.
As for compatibility, make sure that the operator enabled modem service for your SIM as a lot of operators only let customers use their SIMs only in their phones.
Next thing you could check is if the operator supports the modem that we use in our devices (the region to be precise).
You can check what models for what regions we have in our device webpages, under Ordering menu.
And yes, the modem can only have 1 SIM card and second slot could be used for your second provider or for the failover functionality.
EB.
