Stay away from anything that has repeater or extender in the name, go with a WiFi mesh system instead. The WiFi of your existing router needs to be fully replaced and this job is perfectly done by a Mesh WiFi system. They come in bundles and create a stable WiFi mesh network that has great coverage. Plus all WiFi devices seamlessly will pick the mesh router that has the strongest signal, without any interruption.
The best solution in my opinion is from Xiaomi, their Mesh system covers a large area, can be expanded by up to 10 Mesh router devices and can handle easily up to 256 attached devices. Plus each Mesh router has an integrated 3 port Gigabit switch. And this kit only costs less than 130 CHF, offers WiFi 6 and also the latest WPA3 security.
Xiaomi Mesh System AX3000 (2-Pack)