How to login to Cisco ATA 187 web interface?

Question

VoIP

I ask for help with setting up the Cisco ATA 187.
I have never worked with Cisco before, now I have this device on hand and I would like to start using it somehow. The maximum I achieved was to set up the IP address, talk to the iron lady on the phone and be able to access the statistics in the browser.
The key - I can not get into the web interface to configure this miracle. All the manuals that I had to find are sent to 1.2.3.4/dev and mostly refer to the 186th model, to the 187th model I also found a mention of this magic URL. When I open this page, I get this message:

404 Not Found
Filesystem is corrupt.
HTTP/1.0 400 Bad Request Content-type: text/html
400 Bad Request
Request Error.

I just can’t understand, Filesystem is really corrupt and because of this nothing works for me, or is it something wrong and I don’t understand something?
In general, the main task is to connect this device to Asterisk and bring a pair of analog handsets through it to the VoIP network. If someone can help with advice on how to configure it, except through the voice menu (there is nothing to do there except IP, DHCP, VLAN and Factory reset), I will be very grateful.