Depending on your current devices, upgrading to a better Wi-Fi access point would probably be a better solution in my opinion.

A WRT54G is"only" 802.11g

An 802.11n access point should theoretically double your range if you have all 802.11n or better devices.

802.11g is very old so I'd guess it's unlikely you have anything that's 802.11 b/g only. Certainly any tablet/cell phone should be 802.11n or better. e.g. iPhone 4 onwards (which was released in 2010).

OR

Otherwise a simple Wifi retransmitter like you already seem to have should be doable with WDS (which is basically the repeater), but note that halves your bandwith.

You mention your re-transmitter plugging into the router. In that case it's probably an access point in its own right rather then a repeater/retransmitter. Do you have two separate access points setup in your phones/devices?