RED15w (Standard/Split mode) Wifi Access guest only subnet no access to anything but the internet

Hello,

I'm wanting to setup a RED 15w in Standard/split mode back to my UTM 9 for my remote users at our remote terminal sites but have the Wifi setup for only guest access to the internet.  We are a trucking company and have drivers who want internet access in our remote terminals but we don't want them to have access to our internal networks just the internet.  There are no IT types at these sites so a simple plug and play device (hence the RED) is best.  Is this possible and if so how please?!? 

Thanks,

Tracy