Factroy reset sounds a bit of an overkill.
Two things to try.
Ensure you have a Proxy IP Port addresses etc in your APN settings.
And set your own IP address for your wifi connection.
Check APN has a proxy.
Menu
Settings
Wireless & Networks
Mobile Networks
Access Point Names
Your Provider
Under APN it should say Proxy, Port No. etc.
Enter you Providers details. (Sorry, I'm not going to know these, ask your Provider.
For T-Mobile.co.uk it's
Proxy 149.254.201.132
Port 8080)
To hard code an IP address for your wireless.
Menu
Settings
Wireless & Networks
Wifi Settings
Menu
Advanced
Use Static IP [x]
Here enter your own IP address, Gateway, DNS etc.
The above are a suggestion only.