I suggest you follow this step-by-step guide. It is very clear. The current official "installation instructions" are not very clear for everyone in my opinion, and need to be replaced by the ones I mentioned.
However, I do not want to disappoint you, but the difficulties and challenges just started for you with the 7530... Please take a look here and there.
Even if you flash OpenWrt on the 7530, you will find out that you need to juggle with the interfaces, device and vlan setup to get what you want. I simply wanted to simulate a standard WAN port (because the device lacks a physical WAN port) on an existing LAN port, and isolate the traffic as if it was a WAN port (i.e. make 7530 a standard (NOT DUMB) router with firewall), and I gave up in the end, with even considering selling the device on eBay. The configuration of this ipq40xx device is $#..show at the moment (not sure if it will be permanently like that); I have been told that if you experiment with the snapshot build, things are much (?) better, but then you need to figure out how to set up luci without having access to interenet etc etc. Even after you finally solve that, you probably need to do the same process over and over each time you need to update your build, until the improvements become part of the standard release image (next year probably).
Also, another limitation is that the device lacks a physical reset button to restore the device to defaults, meaning that if you lock yourself out while juggling (like what happened to me), you need to flash the stock AVM firmware back using the AVM recovery utility and repeat the OpenWrt flash process again!! Are we serious? Good luck!
P.S.
My personal advice? Unless you want to have an all-in-one modem-router/gateway (xDSL support is experimental with the 7530 now with OpenWrt), if you can sort out the modem part in a different way (i.e. if you have FTTH at home or another good modem in bridge mode), then go sell the 7530 and buy a widely supported by OpenWrt and powerful router. You will be MUCH happier and better off.