Difficult to diagnose the problem without more info.
What device as acing as the NAT?
What other devices are connected to the network?
Is everything wifi or is there a mix of wifi and Ethernet?
Can you see what devices are connected to your network (and their allocated IP addresses) there are lots of tools for this both for computers and smartphones.
Simple solution maybe to just setup your network from scratch if you only have half a dozen or so devices using it.
But to answer your question if you have the privileges to access the network setting in the router/ NAT you can specify IP addresses to certain MAC's. For most users it is simpler though to have then allocated automatically.