Could you check if it is not the browser issue, by trying a different one, clearing cache or trying to configure the device in incognito mode?
is the issue specific to MQTT broker or are you unable to save other configuration changes as well?
I also see that you are using an old firmware version. Is there any particular reason for that? Could you try updating updating to the latest (download link), update with Keep settings option set to off and check, if the issue persists?
If the device still fails to save configuration, I would like you to attach a troubleshoot file to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into the issue.