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.

It is possible (as discovered here)

1. Connect to device's wifi
2. Make note of the DHCP IP address and Subnetmask
3. And use those values with Static IP

It is absolutely possible to work with both the backup camera Wi-Fi and LTE at the same time -- I just did it. The problem of having the connection to my Wi-Fi back-up camera blocking my internet use over the cell towers has driven me crazy. The solution is so easy it is embarassing.

These folks are right. Just follow these easy steps:

1. Connect your iPhone to the Wi-Fi for your backup camera.

2. Go to SETTINGS and select Wi-Fi then select the blue INFO icon to the right of the name of your back-up camera Wi-Fi

3. Make note of both the IP address and the Subnet Mask address

4. Select the STATIC option and enter only the IP and Subnet Mask addresses from Step 3 -- (Leave everything else blank)

Your iPhone will be connected to your back-up camera Wi-Fi and also to the LTE signal at the same time. SIRI and all internet functions (Weather, Navigation, Inquiries, etc) will continue to work with the LTE signal while simultaneously enjoying the safety of your back-up camera.