Troubleshooting WiFi Question

Pleae forgive slightly OT question. I have been having trouble with WiFi and DHCP. The router is the DHCP server which defines the subnet. Several reserved addresses within the subnet are not offered to clients but most of the addresses are available for lease. There is a separate WiFi Access Point (AP). For management access to the AP device I have given it a fixed IP which is one of the reserved addresses within the subnet.
What I wish to know is should I have the AP management address in band or give it an address which is not in the subnet as I have seen some articles referring to out of band addresses being used? Is there a “correct” way of doing this?

My setup:
1 modem-router → DHCP starts at, router has, devices like printer, Chromecast, server etc. have fixed IP addresses < and >
2 AP’s → fixed IP’s and …3, DHCP done by the modem-router.
1 wireless router for guests/customers → fixed IP from outside, on “inside” and own DHCP.

For me, there needs to be logic in the IP addresses. Trying otherwise has always led to trouble.