The Dell guys recommendation is correct if the SLC505 is already networked and has assigned a fixed IP, and you also want to be connected to some other LAN via DHCP (company LAN for mail and/or internet access for example).
Another solution could be that the SLC505 gets connected to the company LAN via a Router.
But if you just want to go online with the SLC505, you dont need to buy another adapter or add any router or anything like that. Just set a fixed IP and Subnet on your laptop in stead of DHCP. The IP and Subnet must match the IP and Subnet as the SLC505.
For example if the SLC505 has IP=192.168.1.20 and Subnet=255.255.255.0, you could set your laptop to IP=192.168.1.21 and Subnet=255.255.255.0.
Then do your programming on the SLC505.
When done, set your laptop back to DHCP so you again can access your company LAN.
You dont have to disable you wifi.
In fact, if you are online with your company LAN via wifi, then you can at the same time be online with the SLC505 via the wired NIC.