I've modeled terrain that was 25 km away from the building without problems. But, ArchiCAD does have problems when things are 'too far' from the origin.
You might try Artlantis (free demo period of the download), which should not have problems - and handles more polygons then ArchiCAD as well, allowing you to bring in the entire distant mountain scape from Google Earth and look out your window towards it, rather than just a cone.
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB