There is the dumbAP guide.
It is describing the steps when connecting to console by SSH, so if you find it hard you can do it from Luci web interface.
Essentially you need to change the IP/mask/gateway/dns of the LAN interface to match the subnet of the router.
Also from the interface page disable DHCP server (tick the ignore interface box) and disable everything in IPv6.
You can skip the step to bridge the wan port if you find it complicated and you don't need it.
Maybe it's me .. but using as Access point it shouldn't have a single network, meaining all devices should be connected as 192.168.1.xx (in order to allow roaming, in case i walk from a room where router signal is stroinger, to another where Access Point have a better coverage) ?