Nat feature is not available on NCS540 at this time Carrier grade nat was only supported on xr asr9000 with VSM/ISM cards

the cards should be the same, -FC is flexible consumption which is licensed based documented here https://www.cisco.com/c/en/us/products/collateral/routers/asr-9000-series-aggregation-services-routers/datasheet-c78-741260.html these are lightspeed ba...

the 24x10G-1G is tomahawk, the 4HG is lightspeed. you can check this guide, it depends on the features and chunks used. https://xrdocs.io/asr9k/tutorials/asr-9000-bng-scale-best-practices/