There is a lot here, but fundamentally, your issue exactly the same as you'll see in this thread. There are solutions in that same thread, too.
I'll comment on some of the other things, too.
This is expected behavior for OpenWrt on the Pi with a fresh install. It is only a single ethernet port which is assigned to the LAN by default. The default state of OpenWrt is such that the LAN has a static IP address of 192.168.1.1 and has the DHCP server enabled -- this allows the OpenWrt device to assign addresses to clients on the LAN as most routers do by default.
You could have also simply modified the IP to a different static address that works on your network (i.e. on the same subnet but not conflicting with any other devices or the DHCP pool). Typically, you should also disable the DHCP server in this case. See info about setting up a dumb AP.
There are many options of how to configure a network device. As I mentioned above, you could have used a different static IP address, or you can also configure your upstream DHCP server with a "DHCP reservation" such that it provides a specific, known, and constant IP address to the Pi. OpenWrt by default has a DHCP client on the WAN interface and a static IP + DHCP server on the LAN -- this is how routers are typically used (although it may be necessary to change the WAN to PPPoE or static IP depending on the ISP).
The Pi (especially the Pi4) makes an excellent wired router (with the addition of a USB3 - ethernet adapter), but it makes a terrible wifi AP. The performance will be poor in this mode when it comes to range and multiple wifi devices simultaneously accessing the network. You are better off with a proper wireless AP.
The rest of your issues (cause and solution options) is in the thread I linked above. Have a read through that and come back with any additional questions.