Faroe Islands map in Gaia

Hi, I've been trying to figure out how to add the official maps from the Faroe Islands to Gaia GPS with the custom layer functionality. I feel like I'm doing it right but I'm not getting anything showing up.

This is their map server: https://gis.us.fo/arcgis/rest/services/topo/topo_20/MapServer

and this is how I'm trying to add it as a custom source: https://gis.us.fo/arcgis/rest/services/topo/topo_20/MapServer/tile/{z}/{x}/{y}

I think the format is correct and the link is valid since if I plug in the relevant tile numbers instead of XYZ I can preview each tile image. For example just by adding this link in the browser: https://gis.us.fo/arcgis/rest/services/topo/topo_20/MapServer/tile/8/2873/5191

Unfortunately, it doesn't show up in Gaia GPS so I'm at a loss as to what I'm doing wrong, or if it's even possible. Any help would be greatly appreciated!

Thanks,
Karl