Give the wireless AP router an IP address “next to” your main router.
I interpret these instructions as referring to the IPv4 address. Is it correct? I change the value to for example 9.10.11.12 The pending changes are listed like this:
I can only access via the original 192.168.1.1 address. When I return to it, the changes are still pending unless I clear them.
The instructions seem clear enough. There is not much here to be getting wrong. I am not sure if the correct item is selected from the menu labeled Device? These are the options:
Contrary to your title, you appear to be setting the LAN IP address.
A few things about that...
I hope you are using 9.10.11.12 purely as an example and not actually trying to use that address in reality. You should be setting the LAN address to work within your existing network range (from your main router), and this should be an RFC1918 address. The address you use should be one that is not currently occupied on your network and that is also outside the range of the DHCP pool (as set by your primary router or whatever device has the DHCP server). Do not use random addresses.
The issue that you're having with the roll-back is addressed here:
fwiw, I frequently encounter the problem where I cannot change the LAN IP address on some devices, because LuCI does not offer 'Apply Unchecked' dialogue box, and so encounter the Rollback message.
SSH into the router and execute these commands to for example, change the LAN IP address to 192.168.1.2, as mentioned in the wiki page provided by @psherman
uci set network.lan.ipaddr='192.168.1.2'
uci commit
/etc/init.d/network restart
DHCP server can be subsequently turned off in LuCI etc.