Sorry about the missing details, the guest vlan is configured with, screen by screen:
1. vlan settings -> Guest
2. vlan -> virtual controller managed / custom with a selected scope. See attached picture for the DHCP setting.
3. security -> the only thing I've configured here is Enabled on encryption with WPA2 selected and a typed passphrase
4. access -> I'm allowing all destinations except for the standard wifi network which consists of the production network
Actually, I think DHCP configuration part is quite confusing and I haven't fully undertood this even after reading the manual, it has just worked in the past.