Can I connect to wifi but still use cellular?
Actually this is possible!...

Use the following instructions:
1. Tap the blue arrow next to the name of the AirPort Express WiFi network to view more options.
2. Select "Static" as the type of IP address, and then enter the following IP address and Subnet Mask :

IP Address : 10.0.1.4 (for example)
Subnet Mask: 255.255.0.0

Make sure you leave Router, DNS, and Search Domains blank! This tells the iPhone that it should use its cellular network to route internet traffic rather than trying to do it via WiFi.