Only suggestion I can provide after reading various forums about this particular router is that it's limited in what you can do and the only true way to resolve it may be to just change to a better router. Then you can remove your extender as well.
There's a huge manual I found and forums I read said that it really won't matter what settings you change anyway as the ISP will be throttling your network no matter what you try to do.
Sources:
http://www.tomshardware.com/forum/45384-43-zyxel-pk5000z
ftp://ftp2.zyxel.com/VSG1432-B101/user_guide/VSG1432-B101_1.1_ed1.pdf
https://forums.anandtech.com/threads/help-with-qos-settings-two-routers.2162030/