It's been said for other Deco models, and I've seen plenty of other threads asking how to do it (only to find that it's not possible).  We need to be able to tag a VLAN ID to the Guest SSID. 

I understand Decos are meant to be somewhat simple and seamless, but the consumers that expect simple and seamless aren't the people switching their mesh into AP mode and hooking it up to their own router and managed switch.  If I can't VLAN tag the Guest SSID, I can't assign IPs to the Guest SSID devices from a separate subnet/DHCP server.  If I can't do that, I can't control how those devices communicate with the internal network in my house.  I want my IoT devices for example to talk to my Home Assistant controller and call out to the internet when needed.  I don't need them talking to any of our personal computers or god forbid, finding access to the router's access page.

I don't think we're asking for a lot here.  I've seen plenty of non-mesh wireless router/ap's that can setup any number of virtual SSIDs with specific VLAN IDs.  That'd be fantastic of course, but we're just asking for the ability to set the VLAN ID for the Guest SSID that's already built in.  Or heck, just hard code a VLAN ID when the user selects the "Isolated from Main Network" option and tell us what the VLAN ID is.