Wifi connection problem

[#1]

Hi, I have a friend staying long term whose Sony Experia won't connect to Wifi using my Plusnet/Thomson router (when he's in bed - he's recovering from an operation!), yet it will connect using the TP LInk range extender when he's in the lounge. He always gets the message authentication problems on his phone when he tries to connect.  Does anyone know how to get this fixed?  I don't have a problem with my smart phone, although mine's a Nokia...
Yours, confused as always,
Jeanners

[#2]

Hi  jeanners49,
Does there phone give any idea what the issue is? i.e is it seeing the router in range or not? or is it maybe failing authentication?