The TC does not have particularly strong wifi.. although you should position the TC carefully, high up in the room to get the best range possible.
To extend the wifi you must use another apple router.. it will not work with another brand. That is what we normally call wireless repeater.
If you have some ethernet installed or would prefer better speeds.. then using roaming network will work even better.. you can choose high rf power devices with better range.. but they must be tied back to the TC by ethernet.
The secondary devices in the case of roaming network do not need to be apple but that article shows how to set it up.