A WLAN repeater is not suitable for achieving better WLAN coverage. I recommend buying a WLAN mesh system. These consist of 2 or 3 devices that build up a network among themselves and can thus bring WLAN to the furthest corners of a flat. WLAN repeaters let the data throughput drop and the connected devices starve at the WLAN repeater until the signal breaks off completely. Then you have to wait until you can connect to another WLAN repeater. Incredibly tedious.

With a WLAN mesh system, on the other hand, every WLAN-capable device grabs the strongest mesh node and there are no more WLAN signal interruptions when you move around within the area. One of the cheapest but still very good WLAN mesh repeater systems is the TP-Link M4, which is available with 3 WLAN mesh repeaters from 130 Swiss francs (also available as a set of 2 for just over 80):

TP-Link Deco M4 AC1200 Dualband WLAN Mesh 3-pack