Can Di2 only be tested as a complete setup?

I’m having problem testing my Di2 setup. Long story short, I got a pair of used but in very good condition Dura Ace Di2 9070 brifters for a cheap price. I then bought a new Di2 cable set with external battery mount, external junction SM-JC40 and the SM-EW90 junction A. For the RD I bought an Ultegra R8050 RD, also in a very good condition since the shop said they just took it off a new bike. The batteries are the SM-BTR1 with around 50% charge, as indicated on the junction box when I plugged the cable into it. Currently I’m missing an FD.

Now the problem is I want to test whether the setup works out of the box without firmware update so I plugged everything in, checked the connection etc, but the shifter isn’t doing anything and the RD doesn’t move. Holding the shifter button also doesn’t show the battery level. So my question is: Is it normal for Di2 to not work without the left shifter and FD connected? Or is it really a firmware problem?

Thanks in advance.