I did this recently on a HooToo TM-02 and there are no instructions for this.
If you are not aware, you should download the packages that match your specific build version. If trunk has been rebuilt since you installed the image you may have problems installing at least kernel dependent packages.
As I see it there are 2 options.
1 - edit your wireless file manually to access your WWAN and OPKG from there
2 - download packages to a local PC, move them to the device and install them from there (wget)
I downloaded all the build's packages and installed Luci and USB (method 2) as I wanted to be able to support the trunk version until a final release is made. Option 1 may not work after the next build cycle. If you had OpenWrt and have a copy of your wireless file (in a backup?), this may work.
I then put the packages for my build on a USB drive and now have a local repository on the flash drive. I do not believe this is an option for your device.
You also will face a space issue with 4mb flash. There will be very little free space for packages.
Also see