NAS540 - Can't reach the webGUI after firmware update attempt.

Hi there dear community members.

We have a NAS540 in our office which we don't use. A couple days ago I decided to give it a shot and found that it was still working. It was running with 5.11 firmware. I thought it would be good if I update it to the latest.. It couldn't find the latest firmware by itself and I wasn't able to find it on the download center. so I searched for a mirror on Google and found 5.20. I updated it manually and now I lose access to webGUI. Looks like everything else operating well but I can't reach the webGUI and that means it is useless for me.

I searched on the community and found there are some rescue disks but the links are broken. so if somebody has those files I would be so grateful. Also, appreciate any recommendation.