Adding airport extreme to existing network breaks wifi

I have gone through a two step change to my network and the last step seams to have made it nearly unuseful.

Step 0:
My network looked like this and have been working for years:

Internet coming in to the house to an edge router that handles firewall and dhcp. It’s connected to an airport time capsule with two ethernet cables going to different floors in the house. At the end of the first ethernet cable there was an old airport express and at the second an airport extreme.

Fiber —> edge router lite —> AirPort Time Capsule (802.11ac) 
(eth0) —> AirPort Extreme (802.11n)
(eth1) —> AirPort Express (802.11g)

Step 1:
I replaced the AirPort Extreme (802.11n) with a newer AirPort Extreme (802.11ac) and removed the airport express. This worked well. I installed the new extreme by resting it and using my iPhone to basically taping next, next, next in the setup process. It’s in bridge mode and creates a new network with the same name (I have automatic on both 2,4 and 5 ghz). I ran this setup for about a week and had no issues except sometimes I had a weaker wifi signal at the spots were the express was located before.

Fiber —> edge router lite —> AirPort Time Capsule (802.11ac) 
(eth0) —> AirPort Extreme (802.11ac)
(eth1) —> nothing

Step 2:
I then added another AirPort Extreme (802.11ac) and my network now looks like this:

Fiber —> edge router lite —> AirPort Time Capsule (802.11ac) 
(eth0) —> AirPort Extreme (802.11ac)
(eth1) —> AirPort Extreme (802.11ac)

I setup the new airport extreme the same way as the one before using my iPhone. But now several of my devices have both issues with getting a wifi signal and when they claim to be connected to the network apps can’t connect. This is for a wide range of apps like safari and social media apps.

I have tried a few different things:

• I have switched places between the new airports but the error still
  remains
• When unplugging either of the new airports the wifi works great
• I have read the different guides at apple (for example
  https://support.apple.com/en-gb/HT202056) and youtube
  (https://www.youtube.com/watch?v=sYEAO9aurxg)
• I have checked that the channels don’t overlap with something close
  to my house

When both airport extremes are on the network my wifi just won’t work reliably. Can anyone help me work this out? Thank you.