I have also tried to inject Cisco Switches and Routers to the SEM but it didn't work.
When I used the flash interface for a Cisco Firewall sometime ago, it worked perfectly ok as I used the automated way. but the HTML interface doesn't have such feature, hence the difficulty in getting logs into the SEM using connectors.
Do you know what version of the SEM you are using?
We didn't introduce the ability to manage SEM connectors via the HTML5 interface until at least version 6.5. You should really consider upgrading to SEM 2020.4 if you're using a version older then that.
