I'm trying to unload drones from my service train into a roboport on an outpost, but only when needed. So I did the following:

I connected the roboport with a red wire to a filter inserter. The total construction bots should be 3, right? And it's broadcasted with the letter T.
On the filter inserter I did the following:

So it should operate when there are less than 20 of T, i.e. when there are less than 20 construction robots in the network. There are 3, 3 is less than 20, but the inserter doesn't move. There is no influence from any other roboport, logistics network or circuit network, I built this some distance from my base to rule anything out. What am I doing wrong?