Is it possible to use custom maps on Sigma ROX 12?

Check out this review: https://www.dcrainmaker.com/2018/06/sigma-rox-12-0-sport-cycling-gps-in-depth-review.html. His in-depth review states you can create custom maps using Komoot, Strava and AllTrails.

Looking through the specs of the ROX12 it should be possible to store custom map data on a compatible SD card.

The device itself seems to use OSM format, this is Open Street Format that can be downloaded from either Open Street Map

Or

More Open Street Map Info

According to the manual ROX12 User Manual you should be able to select custom maps from within the device. You will however need a PC of some sort to download the maps and place them on the SD Card, also note the maximum SD Card is 128GB so you want your custom maps being to big. The advantage of using the first Open Street Map website i linked to is you can pretty much pick any size map, if it gives you an error of maximum export size exceeded you can easily make the map smaller just by dragging your mouse over the page.

Hopefully this helps the OP achieve what they need.