Does anyone know whether the cAP devices act as repeaters?
It looks like the "cAP ac" device does. But I want to be sure. To be clear I just want to expand the range of my current wifi with a device that would not be physically connected to the primary wifi.

It would be working in conjunction with a CRS109-8G-1S-2HnD-IN.

All wireless capable devices with at least level 4 license will act as repeater if you set them so. But think twice about the consequences. The physical laws will stand tall against you.