No internet with NVR/switch between Decos and main router.
I have added a drawing of the intented setup.
I ended up chosing this setup because Reolink support told me that the NVR would double as a switch. This meant that I wouldn't have to place a switch after main router to feed NVR and the 2 Decos.
Unfortunately it seems have no internet connection when I connect the Decos after the NVR 
There might be some kind of Deco settings I am missing? Via the Deco app I thought I found the solution by changing the operating mode from "Wifi router (standard)" to "Accesspoint". Unfortunately It didn't make any difference... Help!
I spend more than a few hours routing cables in electrical pipes across the attic so this better damn work!
Important info:
The NVR (Reolink RLN8) has PoE ports delivering 25W.
Everything is cabled with CAT6.
Deco X50 PoE (Euro) draws 18W and seems like they get power - starts with green LED, but shortly after followed by red LED shortly (indicating no Internet in the Deco app).
Tested Deco where I remove LAN/internet from NVR and inserts it directly into Deco => Internet works so cable between main router and NVR should be fine.
Tested one of the cameras connected to the NVR (with LAN/internet in NVR) and that works fine. I can see the live feed in the Reolink app.
I am currently waiting for an answer from Reolink, but I suspect they will (again) tell me that the NVR (model: RLN8) should work as a switch for the X50's.
In case Reolink has the solution, I will of course post it here as there might be others who might copy the setup.
I think worst case might be that I have to get a PoE-switch after main router to feed NVR and X50's. The NVR is placed right next to one of the X50 so a PoE switch would just be placed here.
Before this setup I just had a 3-pack with Deco M9. Worked fine but I was tired of the adapters blocking 3 visible outlets. Also the X50 Wifi6 capability would fully support the internetspeed we pay for.
I understand that I, with the new setup, maybe didn't have to use a Mesh system, but one of the X50 is actually the X50-Outdoor unit. It's setup outside the house where it covers half the house, the garden + orangery (small glass house). If I finally decide on that robot lawnmower it will also have easy wifi access.
Thanks a lot!
I checked it out at Reolinks homepage and this must be the issue - see screenshot.
Unfortunate that Reolink support didn't ask me to be aware of this when I specifically asked them if I could draw X50 through their NVR.
Now I have to figure out if Reolink NVR will run if it's connected to a PoE switch (to the NVR LAN port)?
The PoE switch I will likely have to add must also support IEEE 1905.1 right?
Yes, like Alexandre said, the IEEE 1905.1 is necessary to establish the Ethernet Backhaul between Decos.
Now I have to figure out if Reolink NVR will run if it's connected to a PoE switch (to the NVR LAN port)?
There was a FAQ shared by Reolink: Method 3 is more recommended.
Best regards.