I'm sorry to hear that you're still having issues.
This could have been caused by software migration issue as you are upgrading from quite old Legacy firmware version and that might affect how firewall rules are configured.
It would be important to determine, whether this is caused by the device firewall settings. Could you please try stopping it for testing purposes by running: /etc/init.d/firewall stop via the command line?
Command line interface can be accessed via router's WebUI by switching router UI to advanced mode in the top right corner and going to System -> CLI and entering login details (username: root; password: password that you set for your router's WebUI).
More information on how to access CLI can be found here:
If this doesn't help, that would indicate that there is a more serious issue and you might have to restore the device to default settings and reconfigure. That can be done by navigating to System -> Backup - > Restore default settings.
More information on resetting here: https://wiki.teltonika-networks.com/view/RUT240_Device_Recovery_Options#Factory_reset
NOTE: This will erase everything on the router!
However, if stopping the firewall would help, then we would need to take a deeper look into your firewall configuration, could you please download the troubleshoot file and attach it to this forum thread?
To download the troubleshoot file please navigate to System -> Administration -> Troubleshoot -> Download Troubleshoot
More information on where to find it:
Note: make sure that you re-enable your firewall once you finish the test by running /etc/init.d/firewall start