Hi Richard
As of the latest stable OpenWrt release (18.06) there appears to be firmware available only for the Bullet M model.
The OpenWrt wiki page suggests that the XW model is supported in snapshot, but it does not seem to appear in the snapshot download page
Assuming that the device you want to use is a Bullet M, then it is supported in OpenWrt stable releases, and in the SECN 4 firmware here:
The catch with the Ubiquity devices is that (sadly) they actively discourage the installation of non-OEM firmware, so you proceed at your own risk and with no support from the manufacturer.
So, assuming that you can get the device downgraded successfully, you should then be able to install one of the stable OpenWrt releases using the "factory" version of the firmware from the download page.
Once you have that running successfully, then you can install the SECN 4 sysupgrade firmware (as you have done previously)
Please note that when you install the SECN sysupgrade firmware, you *MUST* remember to uncheck the box on the OpenWrt firmware Upgrade page that retains the settings.
If on the other hand, the device you want to use is a Bullet M XW, then it appears that the OpenWrt firmware is not actually available for it at this time.
Regards
T