I have two ZSun devices. I successfully installed OpenWRT on one, using the instructions here: https://github.com/brunompena/zsun-resources/releases
Unfortunately, the second one didn't go so well. It appears there's no recovery installed. There is an OpenWRT install and I finally managed to get the password changed (The other one defaulted to no password) and I can access it. How can I flash the recovery?
That device doesn't appear to be supported by the official OpenWrt project. It appears that you have a fork that someone created specifically to run on that device.
You should reach out to the maintainer of that firmware for assistance.
Did you try https://github.com/brunompena/zsun-resources#reflash-recovery-image?
You can also check the zsun thread: Supporting ZSUN Wifi Card Reader (16MB Flash, 64MB RAM, AR9331)