DHCP means Dynamic Host Configuration Protocol and it’s within DHCP that you can configure, among other things, the IP address of you device, the DNS that your devices will use, and some more (I don’t remember which ones since I don’t use more than that).
Having your fixed IP addresses set in a single place has, IMHO, numerous advantages, as centralised management, single place where you can check the IP address of a device and, I think it’s also important, assign a name to each device to be used inside of your local network.
For example, I have my Hubitat hubs with fixed addresses and each has a unique name. Instead of using their IP addresses to connect to them, I use a short name, e.g., h1 for the main hub and h2 for the backup one.
So, instead of going for each device and setting a fixed IP address (some can event don’t give you this option) and keeping tab of the used IPs, you can just set it in a single place.
At your Wi-Fi router you can specify a fixed IP address for each device that you want to. My router, an ASUS with Asus-WRT firmware, has space for 64 such fixed addresses.
And, in my case, since I have all my devices with fixed IP addresses assigned, I can easily identify devices that don’t belong to my network, as a possible hacker neighbour …
So, I strongly suggest that you use your router DHCP configuration and assign the fixed IPs there.