When the DIR-855 is configured to act only as an access point, the unit will not allow entry of an IP address on the LAN, but outside what it considers the LAN.
In AP mode (eg. UPNP and DHCP server disabled, no use of WAN port, static IP for unit), the system cannot know about other LAN routers or the network topology of the LAN, and therefore cannot determine which IPs are LAN vs. WAN.
In my case, I have another router that provides multiple LAN networks (eg. 192.168.10.0/24 and 192.168.20.0/24). If the unit is on the 192.168.10 network, it will not allow configuration to log to a syslog server on the 192.168.20 network.
In LAN mode, the system allow any IP to be entered as a syslog server, and just place the packets on the wire for the other router to handle.