Hi mecaroo, The normal sort of maximum range, quoted by most manufacturers, with a home WAP (wireless access point) that is built in to many routers) is in the order of 100ft (in still undisturbed air). The signal is attenuated by any solid object twixt WAP attenna and the receiving antenna in the mobile device. Walls, machinery, and interference from other devices operating on similar frequencies also can have a significant effect.
Location of the WAP and the receiving antenna can be important and a certain amount of trial and error may be required to find the locations that provides the strongest signal. even a small movement of just a few feet can make all the difference.
If you make sure that the maximum signal strength is being transmitted by your WAP and if its possible to fit a large extenal antena then this may do the trick.... or failing that, a high gain USB Wireless network adapter would probably help. If you used for example a 8dBi directional dish antenna (not terribly expensive) then you could look to increase the effective range by something in the order of 300% or so.
Here's an example of the sort of thing you could try.....
Hope this helps