Lansweeper scans network devices if one of the protocols mentioned in
this KB article is enabled on them. If this can't be done, you might enable an option to create an asset page for IP addresses which were successfully pinged, without retrieving any further data. Under
Configuration\Scanning methods, section
IP Address Range Scanning, tick "Save Ping IP" for the affected IP range.
For switches and firewalls however we would recommend that you check if any protocol can be enabled. Possibly you'll need to enter the corrent SNMP community password and map it the the right IP range under
Configuration\Scanning credentials.