There is no official communication from the company if Nokia 8.1 will be able to work on the 5G network or not.
Yes, you can use 2 4G SIM cards at the same time in Nokia 8.1.
Nokia 8.1 comes with the Android Pie out of the box. You will also get Android updates for the next 2 years.
Yes, Nokia 8.1 does support the FM radio.