To be clear, that is a "factory reset" to OpenWrt defaults.
That is not what the reset does.
Yeah, that happens... it should be on the device info page, but it is true that people get excited to try OpenWrt and don't see that warning. Anyway, don't worry about that.
You should be able to access the device by connecting via ethernet to the router and with the computer set to DHCP. You should get an address in the 192.168.1.0/24 network (with the router reachable at 192.168.1.1). This will be OpenWrt, not the factory/vendor firmware.
To return to stock: