In summary: If an ER-X configured as a switch is to be managed by a PC from a different subnet on an ER-Lite you should
- add a gateway to the system tab on the main page (10.1.2.1 in my situation) or
- add a static route to the ER-X configured as Route Type: gateway, Destination Network: subnet of the PC (10.1.1.0\24), Next hop: address of the router for the ERX subnet (10.1.2.1)
- or perhaps do both just to be sure.
Thanks to all to pitched in to make this clear.