The risk of assigning a fixed IP address (assuming the netmask and gateway are correct, of course) from within the DHCP pool is that while the HDR-FOX (or whatever) is off, the router might lease out the same address to a DHCP request from another device. When the HDR-FOX comes back on line, network conflicts ensue. Just because it normally doesn't, doesn't mean this can't and won't happen (particularly if a router reboot makes it forget its previous assignments).
The best way to assign a static address is to reserve it at the router (identified by MAC), from outside the DHCP pool. The pool can be reduced to a smaller set if necessary. The HDR-FOX would then be set to DHCP, and the router will then lease out the same IP to that MAC each time. Sometimes the router menus call a fixed allocation "infinite lease".
The above doesn't work if the connection is not direct by cable (eg powerline), because the connection through the adapter probably will not have established in time for the HDR's boot-up DHCP negotiation, and then it defaults to nonsense values. Under those circumstances, configure the static address at the router, but also set the same address manually at the HDR-FOX (instead of DHCP).