Wow, astonishing engineering.Polar Express is the big one, goes 12500km around the north and east.
Better one to cut would be Kingisepp-Kaliningrad System which links the oblast with the rest of Russia.
However I doubt that chopping either would have a significant effect compared to cutting transatlantic links.
Submarine Cable Map
TeleGeography's comprehensive and regularly updated interactive map of the world's major submarine cable systems and landing stations.www.submarinecablemap.com