If the router does not have integrated mesh technology, I would buy a mesh system, the WLAN at the router is switched off in such a configuration and the mesh routers then take over the WLAN. You only need to connect the first mesh router to the existing router via LAN cable and then configure the other mesh routers.

Extending an existing router with an extender or repeater is useless because you can't get a better WLAN with different components. This "amplification" is always a mess, because in the worst case you only have half the speed. Today, mesh systems are used to get a very good WLAN.

This Xiaomi AX 3000 set consists of two mesh routers, each with a 3-port Gbit switch, which can manage up to 240 devices and can be expanded to up to 10 devices, I can recommend this set, it is also very well priced and you get very good hardware.

Xiaomi AX3000

Since the photo at Digitec gives the impression that you only get one router, this set contains two identical routers for just under 120 francs.