Can I use SIM other than the Sixfab SIM?
Sixfab CORE solution doesn't support other SIM. For different SIM user should follow the user manual of the mini PCIe module.
How do I use the UART connection?
To use the UART follow the steps mentioned in the UART guide.
Which versions of the Raspberry Pi are Kit compatible with?
The HAT is compatible with all Raspberry Pi which contains a 40W GPIO header.