Hello,
RUT900 is an end-of-life device, meaning that it is no longer being manufactured nor supported.
Regarding the latest legacy firmware update, it is unlikely that the update included any solutions related to your issue, as its primary objective was to provide security patches for a few critical vulnerabilities. This is the case with legacy firmware updates now, to accommodate security updates and occasional hardware related changes.
Please send a troubleshoot file in a private message. It might provide more insight into the issue, though the best solution would be to replace your device to, for example, RUT951.
Best regards,