Hello, my parents have flat and they connect to a router that is floor above. The problem is that the signal is quite weak. What is better solution router or repeater?

I feel like repeater is exactly what they need but looking at repeaters' sizes I doubt they have the same strenght as routers? The device must have function to connect to that router floor above (there is no option to do it via ethernet) and then to send internet via wifi to their devices. Thanks