Weak GPS is a badly worded message.
What it means is No GPS.
The GPS it's referring to is the GPS signal from your iPad.
The Phantom has its own GPS and it's working fine but follow-me requires the phone or tablet to have its own GPS so that the Phantom knows the position of what it is to follow.
If your iPad does not have a GPS receiver or its GPS is disabled, a wifi connection to your phone is not giving it GPS data.