Hello,
I have a modem (O2 Box 6741, static IP: 182.168.1.1) connected via LAN (WAN is without any funktion) to the WAN of the router (with OpenWRT 21.02.1, static IP: 192.168.2.1).
It should just work (with OpenWrt at basic default settings) to connect to the OpenWrt LAN then send your browser to 192.168.1.1. The default route (for anything other than 192.168.2.0/24) of the OpenWrt box is to its WAN port and thus it will go to the modem.
You're using the O2 box as DSL to Ethernet (with routing to 192.168.1.1) then routing that network again in an OpenWrt box to 192.168.2.1, right?
The DSL line is the O2 box's WAN. The "WAN" port on the back is if you had a different type of ISP and not using the integral DSL modem. So leave that port unconnected in your usage.
When you attach the OpenWrt WAN network to the modem, that makes it configure a 192.168.1.X network on the WAN port. You don't have to add another. Do not add another. This is one of those cases where default configuration (other than changing the LAN IP to 192.168.2.1 so as not to conflict with the WAN network) will suit your needs.
First though I would connect the modem (LAN) directly to a PC, let the PC obtain a 192.168.1.X IP, and confirm the modem accepts login at 192.168.1.1.