@TP-Link.De 

Do You have any other mesh devices from Fritz box in the network ? NO

Is mesh on the Fritz box active? YES

Have You tried assigning a static IP to the range extender? NO

Can You please explain the problems that are occurring in more detail ?
All mobile devices don't transfer smoothly. If I walk around and want to change the from the fritzbox range to the tp link range, it still holds the low signal of the fritzbox very long, and vice versa, apprxm. 1 minute, then the wifi icon signal starts blinking and connecting to the tp link, often with a 30 second interruption showing a exclamation point next to the wifi icon. Most of the time, I cut that short be reconneting the wifi period.

The listing of the mesh is not showing the tp link as repeater only as 3 wifi clients 2,4 & 5 & Lan. See picture.

Same appears in the network listing.

I recently even upgraded the firmware to RE650(EU)_V1_201112 but no difference :-(
Funny that the provided mash systems do not support each other, makes no common sense, only if each company want to force their customers to by their very own products only, that's not very customer friendly tho.
It would be great if you guys can provide a work around or even better a working solution, thx

Well unfortunately all nowadays company act the same customer unfriendly way and just want to sell their products period. Also when I bought the RE650 there was no information, that mesh only works with V2. On top TP-Link mesh is also useless until it works with other mesh, like AVM mesh in repeater mode. The biggest disappointment is that in 2021 not even among the devices in terms of connection comes to uniformly good connections. No matter which company, all work only in their own pockets and unfortunately not at all for the customer. I feel dumped because TP-Link does not give clear info on the sales portals about this in advance. If the customer has bought it is just too late, in every aspect. Y'all should think about that