Publisher wants to show popads.net popunder always first (CPM bid doesn't matter)
The next pops from the other ad networks should be shown according their CPM from the highest to lowest after popads.
What you need to do is to add popads creative to the popunder campaign, set Frequency capping to 1:24 (86400 seconds)
and set very high flat rate for it in Advanced settings, let it be $100.
It means that all crawled CPMs will be ignored and popads will be shown first for each unique visitors.
Now your visitors will see popunder from popads first and other networks after.