Most, but not all, do. So it might be as simple as setting a static address, or it may overlap in the future.
You could ask from ISP (or try it out yourself) if you can use some addresses outside of DHCP pool, my ISP router had /24 subnet with .0.1 as gateway but DHCP pool started from .0.101 so there would've been plenty of addresses to use. Mine had a 'end user' account too from wehere I could've changed LAN IP's, SSID and other basic stuff, but I replaced the whole thing with my own.