Yes. If you want to use the UPC box just as a modem and use the ASUS router as your WiFi router, you can do this:
- Connect the WAN port of the ASUS router to one of the LAN ports of the UPC box (any of the ports 1~4 would work)
- Configure the ASUS router in the "AP mode"
- Set any configuration needed for the WiFi on the ASUS router
- Disable WiFi on the UPC box
After these, you can use the ASUS router for WiFi without any issue. For a wired connection, you can connect either to the ASUS router or UPC as with this configuration, from the LAN point of view the ASUS router will act like a network switch.

Merci

Should not be an issue. Ask UPC if they can configure your box as bridge.
Otherwise it can also be run as in normal mode, connected to the box, you can disable UPC box wifi in settings not to interfere.