Had the same problem, the router is a Teltonika RUT240 with the following firmware:
Internal modem firmware version EC25ECGAR06A04M1G
Didn't want to lose the settings as I was upgrading the firmware from a remote location, so restoring the device to factory defaults was not an option.
The DDNS package was also listed in the Package Manager, status was also "In queue" but was not being downloaded/installed.
So I removed the DDNS package from the queue, then downloaded the DDNS package file from the page with the available packages:
RUT240 Package Downloads - Teltonika Networks Wiki (teltonika-networks.com)
and went to Services->Package Manager->Upload and uploaded the downloaded package file.
It got uploaded and installed successfully (installation takes a few moments) and afterwards the DDNS service started working normally again.
The "QoS scripts" package was also in queue, so I followed the same procedure, removed "QoS scripts" from the queue then downloaded the "QoS" package from the packages page in the link above (I checked the contents of the compressed "QoS" package file and among the included files there was also one named "QoS scripts" so I guess that it also got installed, as the package is now listed as just "QoS" in the package manager and not "QoS scripts").
Hope this helps!