WOO HOO!! Turns out it's not a setting. It just works!
Dell Precision 5550 System BIOS has been updated! Version 1.11.0 released on the 7 Dec 2021 has "Added the support for Home and End functions on the left and right keys respectively on the keyboard" But I can't find this in the BIOS. Did I just miss it? Is it called something strange? Dell Precision 5550 System BIOS | Driver Details | Dell Australia
Precision 5550 BIOS 1.11.00 Home / End to fn Left / Right