Oh sorry my mistake, the wifi configuration goes in /etc/config/wireless. I've edited my post.
In /etc/config/network, you need to delete the default Ethernet adapter to allow wifi to serve as your WAN connection instead. You do not need to specify the wifi interface there, placing option network wan in the wifi configuration is sufficient.
In a standard build there should be a wan interface and firewall zone pre-defined even if they aren't used (device with only one Ethernet port). If you don't already have it, add this to your /etc/config/network:
config interface wan
option proto dhcp