IP Reservations for Devices Connected to WiFi Outside DHCP Range

Hey, first time poster here. Had a question that I thought you folks might know the answer to.

We have a bunch of Roku streaming devices we're setting up at my work which work on WiFi only. We are setting up a solid amount of these, 100+. I wanted to make IP reservations for these devices outside the normal DHCP range so we don't eat into a solid amount of our available IPs from our WiFi network. Unfortunately, using the regular reservation setup in our router, it won't let me do a reservation unless I make that range available to the DHCP server.

Basically, we have a network that issues 192.168.1.2 to 192.168.3.255, but I would like to reserve these Rokus in a separate range so they don't take up half a subnet of my available IPs, say 172.16.100.x.

We have pretty decent hardware, Fortinet across the board for our APs, switches, and the router. I feel like I should be able to do this, but I'm not sure how to approach it without adding the other range to the DHCP. Is this even possible or am I approaching this the complete wrong way?

Thanks for reading!