@monahancj 

no it does not isolate from the main network. It is a useless feature.

@monahancj I should have figured out how to test it first.  It seems it is isolated.  I ran a ping scanner from my smart phone device while on each wifi network and only the devices joined to each network are seen by the ping scan.

This is probably how it should be done, but I was hoping not.  My Sonos Play:1 speakers are 802.11g, which can slow down the speeds of the wifi network it's on.  With my previous router I had them on a separate wifi network to avoid that.

At least only devices on 2.4ghz might be affected.

@monahancj 

My testing of the guest network indicated it was isolated.  I switched to the guest network and lost access to shares and could not longer RDP into a PC on the regualr network.

I am on a beta version, 1.4.0 Build 20200319 Rel. 5525.