@TP-Link Thanks for the response!

I got the email from you, thank you. The printer is a Brother HL-L3270CDW, wirelessly connected (to one of the wired nodes). 

When starting to gather screenshots from you, it turns out my iPhone now can find both the Elgato Key Light AND the Printer, regardless of which AP it is connected to. That was not the case yesterday. However, my iPad still experience the same behaviour that the iPhone and my iMac did yesterday. Attaching screenshots for both. No error messages, they just say it cannot be found. 

Moving with my iPad to the same node that the Keylight has, it can all of a sudden find it. So the behaviour that I experienced yesterday on my iMac and iPhone is now gone (as far as I can tell), but it remains on my iPad. Have not tested any other devices yet.

Will test to turn off the Mesh technology for the printer and the Keylight as well. 

UPDATE: Turned off the Mesh Technology for the Keylight, no difference when it comes to the iPad (i.e. it can find it when connected to the same mesh node, but otherwise it doesn't find the item just like in the picture attached.). 

Update after another 24 hours. The same behaviour as yesterday persists. I've also noticed the same behaviour with my MB Air M1. 

So to summarize: 

- iPhone and iMac can connect to printer and to Keylight when they are on the same wireless network, regardless of which Deco node they are currently connected to. Note that this was not the case when this thread started, and it just started working after about 24 hours for both these devices. 

- iPad Pro (2019) and MB Air M1 cannot find neither printer nor Keylight, if they are not connected to the same Deco node. This behaviour persists even 48 hours after original posting. 

During the past 40 hours I have not made any changes, any restarts etc, except for turning off mesh technology on the Keylight as suggested. I was suspecting some sort of cache issue, but since two devices cleared after ~24 hours and two other devices have not cleared for about 48 hours, that seems less likely? 

I'm a bit at loss here. Any help would be greatly appreciated...

  @TP-Link Thanks for the answer. I got the email and responded, as I could then reach both the printer and the Elgato Key Light. 

However, these problems keep reoccuring. As I'm writing this, I cannot access neither the printer nor the Keylight again. This time they are both on the same network AND connected to the same mesh node.