FritzBox and OpenWrt as mesh repeater/wireless nw bridge?

Because I switchted to Mobile Internet Im foreced to use a FritzBox 6850 5G. It has a bad WIFI sinal strength. Since what I read in the documentation I guessed a Mesh Network is not possible with that setup, so I tried to use an OpenWrt router (Linksys MR8300) as an wireless network bridge following the guide It seemed to work (my OpenWrt router is able to connect to the FritzBox), but it isn't recognized as a WDS client on the FritzOs UI and my Windows PCs cannot connect to it. Linux computers seem to be able to connect though.
I followed exactly the guide at the above page, and double checked it. So I'm asking, if it is possible at all, setting up an mesh network/wireless nw bridge with this setup?

Regards - ch2

WDS/ 4addr only works if both (all-) involved APs have nl80211 based (mainline-) drivers, which the AVM Fritz!Box 6850 5G most certainly does not (they will be using QCA's proprietary drivers).

Thanks for your answer. So there is no way to use my OpenWrt Router as an wireless range extender?

Not using WDS/ 4addr, relayd or a routed subnet would be options - but relayd is rather buggy by design (no IPv6 support either; but this is what commercial wifi extenders do in principle) and a routed subnet a bit less transparent.

Since relayd didn't work either I tried something different. I set up my FritzBox with local ip adress and switched wifi off.
I connected the Linksys MR8300 (OpenWrt) on the wan port to the FritzBox and use it as router for my local network, wifi, dns, dhcp. So far so good, everything inside local network works as expected, internet connection too. But the wifi signal still is bad in some areas. So I want to use a RPI 3B as a mesh node, repeater or wireless nw bridge. But before I waist to much time I wanted to ask if this is possible at all?

With an RPi, no - brcmfmac doesn't support STA and AP modes at the same time.