TL-WR850RE V1.2 - Devices not reachable in 1 direction (dumb layer 2 repeater)

Hello, I installed the firmware listed below on my TL-WR850RE:
Powered by LuCI openwrt-18.06 branch (git-20.319.49209-ab22243) / OpenWrt 18.06.9 r8077-7cbbab7246

I created:
1 Interface as WDS Access Point protocol: unconfigured
1 interface as WDS Client protocol: unconfigured

Both are unconfigured appart from the WiFi access credentials and disabling "builtin IPv6".
On eth0 i created a bridge to the WiFi interfaces and assigned a static IP, so I can access the OpenWrt LuCi.

When connected to the access point, I can reach without any issues:

  • All devices in my home /24 subnet
  • via the Gatway (fritzbox router)

When I am connected to the gateway WiFi (fritzbox router) instead, I cannot reach any device, that is connected to the repeater.

The one and only firewall rule is set to accept all.

What realy is strange is:

  • All is configured as a bridge (OSI layer 2?) - why is IPv6 not working on devices connected to the repeater?
  • If it is all bridged, why can I only access IPs connected to the repeaters AP interface, when the request comes from a device connected to the same AP?

I disabled ALL DHCP settings. I want it to operate in a super-dumb Layer 2 way. Why is this not working and just letting traffic through in every direction?

Sorry for being annoying, but is there really no one who knows the answer?