and was recommended that I get a cable from my phone to the remote control for a better connection.. When doing this do I still need my cell phone? What If I have a wireless ipad mini? Can I use that? Also can I use the mini if I am not connected to any wifi connections if I am using the cable?
Yes.
An OTG Cable can be used with most devices including WiFi only device's.
I fly with a WiFi only mini4 and an OTG cable really helped with a solid connection.
This is the one I use.
There is a slight disadvantage to using a WiFi only device. Since the WiFi mini4 doesn't have a GPS, you can't use the "Find my Drone" or reset the Home Point to you and not the original launch spot. This would be used if on a boat for example and wanted to reset the HP so it won't go back to where the boat was when launched.
I've been flying with the mini4 for over two years and haven't had a problem.