Upgrade script cannot find newer release

One of my servers is unable to upgrade from Ubuntu 12.10 (Quantal Quetzal) to newer versions:

$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=12.10
DISTRIB_CODENAME=quantal
DISTRIB_DESCRIPTION="Ubuntu 12.10"
$ cat /etc/update-manager/release-upgrades | grep -v '^$\|^\s*\#'
[DEFAULT]
Prompt=normal

(also tried with Prompt=lts)

$sudo apt-get update
...
$sudo apt-get dist-upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
$ sudo do-release-upgrade -d
Checking for a new ubuntu release
No new release found

Tried to change servers to http://old-releases.ubuntu.com (got from here), and from digitalOcean but result is the same.

Eventually I upgraded this server using the following steps:

1. Backup /etc/apt/sources.list file


2. Changed original servers to old-releases.ubuntu.com


3. Changed all quantal to raring in /etc/apt/sources.list file


4. Updated package lists (apt-get update)


5. Updated packages (apt-get dist-upgrade) - everything completed smoothly


6. Rebooted server


7. Checked for new distribution (sudo do-release-upgrade -d) - still not found


8. Changed raring to saucy in /etc/apt/sources.list file


9. Repeated steps 4-7 with same results


10. Restored backed-up /etc/apt/sources.list file


11. Changed saucy to trusty in /etc/apt/sources.list file


12. Repeated steps 4-7 with same results

Now I have Ubuntu 14.04 LTS (Trusty Tahr) release. It is OK until 2019-04, but it still cannot find upgrades.

Is there a way to restore the ability of an Ubuntu distribution to upgrade using the do-release-upgrade script?

Command line solution is highly preferred.