weak? wi-fi signal

I use the router supplied by Talktalk for my internet, which works ok as long as I am in the same room with my laptop or tablet. The problem is, my desktop PC is in my old darkroom, which itself is built in the garage, and although only about 4 metres away, the signal has to pass through a brick wall, of which the cavity has been filled with insulation.
The PC is an old pre Wi-Fi model running XP, and to receive Wi-Fi I have a belkin " dongle" fitted, and although a signal is detected by the belkin device, it only runs at about 5 mbts, and as soon as I attempt to access a browser the connection cuts out. I cannot move the PC as I have a scanner and printer attached, and I can't move the router closer to the PC as there is no telephone socket in the darkroom, and as Talktalk refuse to send me a new router, I need to find a way of extending the range of the signal. I have looked at items that are supposed to extend the signal, such as ones which use the existing electrical sockets, but they seem to need to be fitted directly into a mains socket, which I don't have access to, and apparently are not successful when used on extensions, anyone have suggestions as to what might be available to solve this problem?
Thanking you in advance.

weak? wi-fi signal