I try to insert a SIM card in my modem, but it fails to read.
There could be two possibilities for this error:
The modem is not unlocked, which means it's compatible with selected SIM cards from various carriers. You will have to get in touch with the seller in order to obtain information regarding SIM compatibility with available networks.
The SIM card is magnetized which makes it impossible to be recognized by any device that supports it. The carrier will provide a replacement SIM for the one you already have.