Main router is D-Link DIR-850L stock firmware from ISP with IP address 192.168.1.1 (from landlord room).
My router is WRT1900ACS with WAN IP 192.168.1.131 (DHCP leased from Main router).
I set my router LAN IP as 192.168.2.1 static IP and 192.168.1.1 as gateway.
I connected landlord's router (D-Link 850L) to my router's (WRT1900ACS) internet port.
Internet is working fine as usual, but I cannot access the main router (D-Link 850L) webUI now (192.168.1.1) for whatever reason.
Tried to ping/nslookup main router (192.168.1.1) and the result shows that 192.168.1.1 is owned by OpenWrt.lan which is confusing. I know 192.168.1.1 is the main router (D-Link 850L).
When your WAN is DHCP you should not set a gateway or DNS in the LAN section. The gateway will be configured automatically. If you want to use different DNS servers set them in the WAN section.
Are there any other OpenWrt devices on your network?
Your browser wants to redirect to LuCI because at some time in the past you have accessed an OpenWrt router at 192.168.1.1. Now you are reaching the neighbor's router upstream, but since it is not OpenWrt, the LuCI page does not exist. But the 404 error means that you did reach a web server.
You have basically bridged the two lans.
I don't want to imagine what else is wrong there, so better restore factory defaults, change immediately the LAN interface IP into something else not conflicting with your landlord's router and then connect it upstream to his router. It will work just like that.