Just to confirm - do you have internet connection on your device, including properly configured DNS servers on the WAN interface? Any issues related to package manager usually occur due to no internet connection or access to DNS servers.
Instructions on how to login to the router via CLI (SSH/Telnet) can be found on our wiki here: https://wiki.teltonika-networks.com/view/RUT240_CLI and here: https://wiki.teltonika-networks.com/view/Command_Line_Interfaces#SSH
To confirm whether you have internet connection, you may try to login to the router via CLI (SSH/Telnet) and issue the following commands (interrupt the ping sequence by hitting the CTRL+C buttons on keyboard):
If both of these confirm that there's internet connectivity and the domain names are being resolved (DNS works), another method to potentially resolve the issue would be to issue this command via CLI manually:
After issuing this command the router should update its list of available packages. Once that's done, re-check if the package manager has changed.
If that doesn't help, by far the best and fastest course of actions would be to reset router to default settings and see if that helps.