I have three  xe75 and I have the same problem with guest network. Newest update brought IoT network, but unfortunately IoT network even provide also no possibility to set VLAN tagging.

@tplink your Decos are not cheap devices, and basic feature as VLAN tagging for guest and for IoT must be included. 

  @mligor Agreed. The lack of this capability is ridiculous especially considering how expensive these units are.

Folks, if you've stumbled across this thread looking for how to enable vlan tagging on these units, don't forget to upvote this feature request.

It may be too late for me, but I know there are folks out there that are still looking to use this with these APs.

  @tt75 I need this feature too.
Very disappointing this basic feature is missing.
I have a wired network on one subnet, and a wireless on another subnet, and the guest mode allow access to the wired network despite local access is disabled. 

  @tt75 Could you update the title please as this is not just the M9+ but is all Deco. I have X68 & X50. Thanks

Good thing my system is still in the return window and that I will get full refund.

Was planning to use this as AP with pfSense... but will look into another brand.,

Yeah we need this.

Thinking of a workaround, but we will be wasting deco units. What if we create a new network still under ap mode and then assign a different untagged vlan to it in your switch, with a different ssid. We can connect all IoTs in there, together with its own Guest network. Trunk it to your router with its own subnet and policies to block it from accessing your main network. Ensure to run wireless optimization every week or so. Your thoughts?

@TP-Link_Deco Wow the surprise i felt when i saw that an expensive mesh system does not even support vlan tag, and then that they ignore the request on the forums. Never buying tplink again. Cheers.