passport wifi won't connect

For the past few months my Passport won't connect to any Wifi or hot spot. It sometimes shows the symbol but isn't connected and says limited connection. On bad days it won't connect and switches it's self on and off continually. I have reset the phone 3 times and makes no difference and reset it without reinstalling the back up which makes no difference. I used a different network simcard and no difference. What could be the problem? Would really appreciate some ideas as this is spoiling a great phone.

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