if the single IP (12.34.56.2) is RFC 1918 address, the ISP Modem automatically takes care of NAT.
If this is a real IP address you need to contact ISP, is this IP DHCP or an allocated static IP address for you?
based on the answer from ISP you need to change (if ISP looking to change) - if this is setup to DHCP then you need to worry that automatically get new IP address
Note: this depends on the ISP offers.