The easiest thing is to just to use the main router as gateway to the internet and do not directly connect other hosts to this router.
Then you reset the AP to standard configuration. And plugin a cable from main router into the wan port. Be careful, that the two resulting networks are different in IP address space. Now you can do that adblock stuff. 
Further, look at such things as Nat Offloading, that speeds up the OpenWrt router.
[Main Router]<---10.0.1.0/24 <---> [OpenWrt] <---10.0.0.0/24 ----> (Main Network)