What I think I know: a dumb AP needs to use something like /etc/ethers to map MAC addresses to hostnames so you can see them in the associated stations view.
I recently cleared /etc/ethers on my dumb AP and noticed that of all but one of the associated stations changed to a question mark. My iPhone is displayed as its hostname and its IP address it got from the router somehow. My question is how?
Clients usually announce its name to the DHCP server itself. But only if the device is configured to do so.
The usual way configuring static names within OpenWrt is using dnsmasq (static leases, hostnames).
If you edit and delete things in /etc/ethers you have to re-edit the file again to make it work ofc. Ethers is not a file handled by OpenWrt overlay. Same applies to /etc/hosts.
Normally, if you edit /etc/ethers you do the same for /etc/hosts to have a reliable and consistent configuration. That may explain the question marks.
E. g.
/etc/hosts:
192.168.1.100 myhostname
/etc/ethers:
11:11:11:11:11:11 myhostname