Hi there,
Point-to-point interface can always be unnumbered as it is responsible for point-to-point connectivity.
The IPs that are assigned by your provider to you to be used as LAN IPs at your office are configured on your router's Ethernet interface from your side, and from the provider side the simple setup is that the provider adds a static route for these LAN IPs pointing to your router in order for the packets to be routed back to your router from the internet (your router should have a default route towards the provider to send the packets to the internet in the first place).
HTH, Please rate if it does help,
Mohammed Mahmoud.