You should not assign an internet nameserver on the lan interface, but on the wan, where it is reachable from.
This is not the last address but the amount of addresses of the pool.
The dhcp server should work out of the box as soon as you first boot OpenWrt on the lan interface, which by default is eth0 on RPi4. So you can try to restore to defaults and try again, however I'd advise to use the community build.