Pretty much any unmanaged gigabit switches will work fine. 

Just make sure to attach the switch to the w21 and not to the modem. 

Thank you. Good to know not to connect to modem as that was my plan.

Ok I'm home now. Do I connect S33 to WAN port on W21? Then LAN port on Same W21 to switch. Do I need to change anything in Surfboard Central app?

Yes. That's how you do it. Shouldn't need to change anything if you have an unmanaged switch

All connected and working well. Thanks for your help.

Glad its working!