@AdrianHi 

to makes some things clear, I use a pi-hole as dhcp server and in router mode I can see the guest option but not the isolate from main network option, I can see this in ap mode but then I don't have a internet connection, I get an ip adress tho, but in the same range. 
so when I put the m5 in router mode I disable the dhcp server of my pi-hole but then the devices connected to the guest network can still see the rest of my main network and even connect to my server for example. 
 

tl:dr 

my m5 in router mode no isolation from main network for the guest WiFi. 

in ap mode no internet for the guest network. 

@Machielj 

Hi, the guest network and main network are separated from VLAN and they would still share the same IP range as the Deco DHCP server;

So, if you found that the main network is not separated from the guest network on the wireless router mode, please help me check the following information and we would be glad to forward your case to the senior engineers:

1. What is the current firmware version of your Deco devices;

2. Please provide us a detailed picture of your network topology;

3. How did you find out these two networks are not separated?

You could try to leave me a private message about the answers or send an email to support@tp-link.com with [Forum ID 162818]Deco M5 guest wifi isolation；

Thank you very much.

@TP-Link 

Hi there,
Thank you for the information that is provided until now. I would like to confirm that everything is working as it should and will supply detailed information about my setup and the steps I took to verify this here:

I have 3 deco M5 units in my home. They are in AP mode. My Moden/Router is the router. I have NAS1 wired to the moden/router with a fixed IP address. I have NAS2 wired to my deco M5 main unit (in the 2nd RJ45 port) with a fixed IP address. I have a wireless network enabled on the deco m5 units. I also have a wireless guest network enabled on the deco m5 system with the "isolation from main network" option enabled.

When I have my laptop on the wireless network from my moden/router I can connect to both NAS systems through the external IP address, the DDNS address & the internal network address.

When I have my laptop on the wireless network from deco m5 I can connect to both NAS systems through the external IP address, the DDNS address & the internal network address.

When I have my laptop on the isolated wireless guest network from deco m5 I can still connect to both NAS systems through the external ip address and the DDNS address. But I CANNOT connect to either NAS systems (or my modem/router) through the internal network address. I can still access the internet as I am writing this message when connected to the guest network.

In my opinion I can now conclude that, at least for me, everything is working as it should.

If there are any steps that I have missed out on, please let me know.

Thank you very much, keep up the good work and stay safe,

Nick