WiFi mesh routers can be used with any existing modem or router. The mesh router takes over everything, WiFi, the LAN connections, the assignment of IP addresses with a built-in DHCP server. The WiFi of the existing router and modem must be switched off and also switched off, because the mesh router takes over (it is better in any case). Of course, this means that the first WiFi mesh router must be located near the existing modem. Everything existing that can use WiFi must now be removed, no old repeaters should be used (they are not WiFi 6 anyway).

2. one of the WiFi Mesh Routers is connected directly to the existing modem or to the existing router via LAN cable, this is necessary so that the Internet can reach the WiFi Mesh Router. What you should not do, if possible, is leave devices connected to the existing router, because the WiFi Mesh system will provide a new IP address range because it now takes over everything. Everything that needs a LAN connection should therefore be connected directly to the WiFi Mesh router. They usually have several LAN connections with Gbit Ethernet and if it is not enough, simply connect a switch.

And another tip to optimise everything, there are also WiFi 6 mesh routers from other manufacturers, where you can get 3 mesh router devices for about 350 Swiss francs, i.e. you can get further with them. Expanding the Asus WiFi 6 quickly becomes expensive if two stations are not enough. A TP Link X20 WiFi 6 mesh router system with integrated antivirus and antiphishing HomeCare solution costs 359 francs at Digitec with 3 units.

Tp-Link Deco X20 Set of 3

And the even better Deco X60 is only 339 francs (at least at the time I wrote this answer):

Tp-Link Deco X60 WiFi 6 Set of 3