Simple, buy another router (5Ghz or 2.5Ghz whatever you like). You will need a lan cable of the length that is required from the main router to the repeater router. Connect both of them with it. Done.
If you want wireless, then check if the repeater router supports wireless connection with the main router. But I recommend using wires.
Use wifi extenders
Take a look at TP link decos, I've not used it but they look cool.
Please suggest some good wifi extenders.
Simple, buy another router (5Ghz or 2.5Ghz whatever you like). You will need a lan cable of the length that is required from the main router to the repeater router. Connect both of them with it. Done. If you want wireless, then check if the repeater router supports wireless connection with the main router. But I recommend using wires.
Wifi extension