BalenaFin 1.1 and SIM cards (LTE connectivity)

A bit confused here.

The BalenaFin 1.1 has a SIM card slot. If I want to get 3G or 4G/LTE connectivity in EU, do I need to additionally purchase and install Quectel LTE Module EC20EA-MINIPCIE, or will it work in some basic functionality also without the extra card?

I understand your confusion. Looking through the balena fin 1.1 datasheet I found the following

29 nano-SIM socket This allows the use of a wide portfolio of cellular modems via the mPCIe socket (32) 

This indicates that you still need to supply a modem to use 3 / 4G or lte connectivity.