Dumb Question On Fixed IP WiFi Devices

I am having to rebuild my site lan and wifi setup due to device failure and wish to get advice please on a basic principle as I wish to depart from what I have done in the past.

I prefer to set fixed IPs to wifi routers and bridge devices so I may easily access them. Must these addresses be in the same subnet as that of the dhcp server which defines the subnet?

For example I have a firewall/router which is also acting as dhcp server. This is connected by lan to a dedicated wifi bridge (using WDS). The remote bridge device is connected by copper to a wifi router which is accessed by various clients.

The two bridge devices and wifi router all have fixed IPs but I only need to access them to change settings. Other than facilitating this access, is there any reason why the subnet being served by the dhcp server must be the same as the device addresses. I ask because I want to set up the devices and then plug the end into a different router which will also act as dhcp server but will use a different subnet.

Hope this is clear and sorry if question is dumb but I have conflicting advice from hardware suppliers.

Same subnet, yes - but outside the dynamic dhcp pool.
Instead of configuring the IP addresses statically, I'd suggest to just create fixed dhcp leases/ reservations (also outside the dynamic dhcp pool) instead, this gives you the flexibility to renumber your network as needed while also providing fixed local DNS resolution.

Understood. Unfortunately I cannot configure my dhcp lease reservations so I set things up on fixed IPs and exclude those addresses from the dhcp pool as you suggest.
Because none of the wireless devices are being accessed by traffic, once set up, the system works on other subnets. Have it installed like this and all is going well so far.
Many thanks for the reply.