In the portal, goto the addressing & vlans section.
Then part way down the page there's Routing, make sure use vlans box is checked.
Click the add vlan button and add a vlan/subnet for your local network.
Once you've added the vlan, goto the Per-port VLAN Settings section of the page, (just below the routing section)
Check the boxes for the ports you want to use for your local LAN. Hit the edit button. Set the port type to "access" and set the vlan to the vlan you just created.
When done hit the save button at the bottom of the page.
When you pick a subnet for your local LAN you will want to use something that your company does not use.