Well, you can't, so simply the WiFi 6 network can only be extended by adding an additional WiFi 6 capable mesh router device. Buying a repeater is not an optimal idea, because repeaters are an outdated technology. Today, you are better off with a WiFi mesh router system. The best solution is to buy an additional WiFi 6-capable mesh router, but it depends on the manufacturer. In general, WiFi 6 systems can be expanded with additional mesh router components from the same manufacturer. The manufacturer's website should have information on this, e.g. TP-Link offers possibilities with the Deco System to expand a mesh system with additional components, which can then of course only support WiFi 5, but that would not be ideal either. If WiFi 6 is used as the main network, all WiFi mesh router devices used with it should also support the same standard. This makes everything much easier.