The Mikrotik Rb951ui-2nd port LED indicator is off and not detected after upgrading the router but it can be seen on the control panel that the connection is disconnected and reconnected repeatedly
This indicates that RB might be in reboot loop. One probable cause for it is failing power adapter, so I’d first try with a new power adapter.
Next would be to try to netinstall the device. Beware that netinstall is very fragile process and it’s really important to follow the instructions … and it can still fail for various reasons; netinstall completely wipes device and installs ROS anew … it might keep the configuration but there were cases where problems in configuration caused boot loop, so it’s good idea to wipe the config as well and create required configuration from defaults.
Another possibility (since we’re talking about device which is probably pretty old) is that some internal components failed (there were cases where internal capacitors bulged, causing random problems. When these were replaced, device operated normally), but reported cases were different device model, not RB951Ui. So this idea might not apply.