I seem to have lost Secondary Power status, how can I earn it back?
Secondary powers are countries from 9th to 16th place. To improve your status, you just need to increase your points. If you have a big budget, invest in new factories and recruit soldiers. It would waste money if compared to regular development but if you really need points, that's the easy way.
The easiest way to get prestige points is to participate in wars. Unfortunately, this is also a pretty easy way to lose prestige points, so make sure you can actually win before you declare war.