According to the technical specs Nokia 8 should be able to aggregate 3 LTE-carriers, reaching this way up to 450 Mbps in download.
I have a Nokia 8 dual and an Xperia XZ dual, they have similar specs (3 carriers).
Unfortunately the Nokia 8 does not include LTE band 20 (800 MHz) in carrier aggregation. My provider mostly uses bands 3 (1800 MHz) and 20. On a few spots, also band 7 (2600 MHz) is available. There the Nokia 8 uses carrier aggregation.
My Xperia XZ works correctly, uses CA with bands 3 and 20, as activated by my provider.
This must be a bug, and I hope the problem is solved in future firmware versions ...