@Stary If you're using Smart Connect wireless feature disable that and change the 5GHz wireless channel to 36 - then test again:

@Gangadhar1 If your RE is HW v7 (that's on the botton sticker) update its FW to the latest FW release - TL-WA850RE(EU)_V7_1.0.13 Build 231115 - here. Test again and check if there's an improvement.

@EsDeeEm To avoid the double NAT you could also let your Fritz work in modem router mode and configure AX55 in AP mode (Case 1) - try this and check if there's any improvement.

@LeZs You should probably reconsider the equipment of choice. For a 3-floor house whith 700m2 garden there's an option to use a Deco whole home mesh system like Deco X50-PoE. For the garden you could...

@sunil012 You can click on downside arrow and check for which router models this guide applies to: It apllies to Archers models from AX, C and A series.

@Emix86 Check the negotiated speed between your main router and the RE700X. This could be done from your main router WebGUI. If the speed is 100mbps that's the culprit. You may change the main router...