What ISP Service do you have? Cable or DSL?
What ISP Modem do you have? Stand Alone or built in router?
Ensure you have the email addresses and account information setup correctly on the iPhone.
Ensure DNS IP addresses are being filled in under Setup/Internet/Manual?
Turn off Advanced DNS Services if you have this option.
Turn on DNS Relay under Setup/Networking.
Setup DHCP reserved IP addresses for all devices on the router.
Ensure devices are set to auto obtain an IP address.
Set Firewall settings to Endpoint Independent for TCP and UDP.