You will need to look at your main router's DHCP reservation feature (assuming your main router is the DHCP server) -- you'll use the MAC address that shows up for your OpenWrt device and then tell your DHCP server to always assign a specific IP.
It shouldn't... but you'll need to make sure you know what IP address is assigned, of course. Your main router/DHCP server's leases (or the reservation you set) will obviously help you figure that out.
You only need to do this if your OpenWrt router has the lan interface configured as static... you'll see it in the lan interface configuration > Advanced Settings > Use Custom DNS servers
Meanwhile...
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