Did you update the firmware with 'keep settings' turned on by any chance?
Also, make sure you have advanced mode turned on in the upper right corner of the WebUI when you cannot find some of the options in the WebUI. However, the MQTT option should still be visible in basic mode.
First, I would try to clear the browser's cache or use another browser to see if the MQTT option appears.
Then, try restoring the device to factory default settings in System -> Administration -> Backup.
In case restoring to factory defaults did not solve the issue, I would suggest reinstalling the firmware via the bootloader procedure as described here.