Going to fibre shouldn't in itself make any difference to connectivity in the house. That just affects the speed to your front door, so to speak. Everything internal is handled by the router. Just for clarification, are you saying that the fibre speeds are lower than expected or is it your wifi that you are having issues with? I.e. does anything physically wired to the router experience decent speeds? And are you getting the speeds as advertised?
If the connection itself is OK, then it may be an issue with your router, which could include physical placement, especially if previously you had no issues. If you have everything in the same locations as when you had ADSL but you are experiencing a poorer service, that would seem to be the logical conclusion. Do you have access to another router to test whether this is indeed the case?
Before spending money on a wifi extension, just check that it is indeed the wifi that is your bottleneck. You could test wireless connection in the same room, one room away etc. If it's still poor in the same room, then check whether switching channels might help, as you may be experiencing congestion. Most routers are set to auto channels, but you could try experimenting to see whether setting the channel manually might help. The ones to try are 1, 6 and 11.
