Mesh OpenWrt and Fritzbox


I'm new and I hope there is not another thread with the same topic, but couldn't find anything.

I have 3 different router. Fritzbox 7590 (orig. Firmware) and 2 TL-WDR4300 (OpenWrt 19. 07)

The dream wlan setup:

Internet/Fritzbox 7590(Mesh Master) <---->TL-WDR4300<---->TL-WDR4300

My problem is that I want to build a Wlan mesh over this 3 routers, but I don't know really how.
My newest approach is to try it with the 802.11s but if I connect my notebook with one of the TL-WDR4300, I can't communicate with the Fritzbox and vice versa.
I think my knowledge about OpenWrt is also a fundamental problem. I tried different tutorials but every single one with the same outcome (see above).

I hope someone of you could help me

Currently you are just connecting all 3 routers over wifi?
Since I suspect the stock firmware of the fritzbox to not play well with the other two devices my question is what you can / can not ping? Do the 2 WDRs "see" each other?
I have done some meshing including a fritzbox with OpenWrt on it and used batman-adv. That would be a guaranteed working approach. But it takes some efford to understand and set up for the first time.

If the notebook is connected with TL-WDR4300, then I can't ping to the Fritzbox and obviously no internet connection.

If the notebook is connected with the Fritzbox, then I can't ping to TL-WDR4300 and I'm connected with the Internet.

BUT the TL-WDR4300 is connected (over Wlan) to the Fritzbox. This can be seen on both configuration pages.

Yes, so layer 1 connection is working. Which is good. To clarify what I actually wanted to know: can you ping one TL-WDR4300 from the other when they are both in wifi mesh range to each other.
trying to figure out if OpenWrt devices or the fritzbox are the issue here.