Someone provided information on setup. Do you need more details?
Totally confused now. You don't usually have to open a port to make a cellphone work. Nor does an open port relate to your second point.
- Step 1 - follow Post No. 2 above to create the Interface/DHCP
- Step 2 - create an SSID for WiFi (optional)
- Step 3 - place interface in a new firewall zone that does not permit forwarding to WAN
- Step 4 - install and configure a file sharing server
Simple.
You could also edit LAN so that it doesn't forward to WAN - and only allow the IP of the cellphone - although, you may have to configure a static address in the cellphone's network/WiFi settings.
EDIT: Duplicate of - Using OpenWrt, SSH, and network config