A single router is not a good solution for improving WLAN coverage. I recommend using a WLAN mesh router system. It consists of 2 to 3 separate WLAN routers that build up a WLAN network among themselves, also called Mesh WLAN. This works without any problems and provides excellent WLAN coverage. You can then set up the individual Mesh WLAN routers starting from the fibre optic distribution box and distributing them throughout the flat to create a wide-ranging network. Since the Asus router can do WiFi6, I would recommend this system from TP-Link with WiFi6 as an alternative. TP-Link X90.

It has a fast 2.5GHz network connection, which is then connected to the Swisscom router. The WLAN of the Swisscom system is then switched off and the TP-Link Deco X90 then takes over this task.

TP-Link Deco X90 (Pack of 2) AX6600 Whole Home Mesh Wi-Fi 6