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.
I found the Reolink FAQ you refer to:
3 methods (NVR vs. PoE switch) / Reolink
I have asked Reolink about method #3 while adding X50 Deco. They try to be helpful but it is clear that they are not that deep into network wizardry as you guys. They probably never had to try to support someone who wanted to mixup surveilance and mesh 
I was thinking that the PoE switch has to be IEEE 1905.1 compatible for the Decos to create their magical field of WiFi throughout my house. I found this 2 year old Reddit thread (link + a segment of the text I especially noticed below).
Link: Reddit thread about Deco vs. Switches
"In that thread, contributors noted that the official specification of IEEE 1905.1 explicitly states that no modification or special "magic" to enable IEEE 1905.1 should be required on existing switches. This is why you won't find any mention of IEEE 1905.1 support in data sheets for network switches. And indeed it should make sense that as an L2.5 protocol, *every* switch should work, because by definition all switches operate at least on L2. Yet here we are, having to trial and error."
I guess I then at least don't have to worry about the if the NVR is compatible to the switch (since the switch isn't going to be IEEE 1905.1 only). What I do have to worry about is not buyind and returning 10 different switches that won't do the trick!
A colleague of mine has a 30W Poe Injector and also a 65W PoE switch (4xPoE and 2xUplink). I will borrow it the next couple of days for testing 
Last update and thanks again!
Above is mail from Reolink support.
i still don't fully understand why it's not better to plug the cameras directly into the NVR (which is also PoE) since the NVR is optimised for cameras. I'll plug it into the switch for now though.
Just ordered a decent switch (1000mbps) so I get the speed we are paying for :)
The switch I borrowed is only 100mbps max.