Sonoff switches for 2way circuit?

I wanted to put smart switches on my stair light so I bought Sonoff ZBM5-1C-86W and ZBM5-3C-86W.

Here's some context:

• UK 1960s original wiring
• Downstairs has permanent love
• Downstairs is 3 gang ( front porch, hallway and stairs lights)
• Upstairs is 1 gang (stairway light)

I have installed them (Don't know if I got it right) however I had a thought. If the circuit isn't complete then how will the upstairs switch ever get power?

Am I being dumb? Don't I need a bypass? Or is it that I've just bought the wrong product?