I spent hours trying to get this to work. I had to do this due to an IoT which has to be connected LAN. I thought I would share this for anyone else that wants/needs to do this.
I recommend you backup the Flint 2 and do this at your own risk.
Login to LuCI
Network > Interfaces > Devices (tab)
br-lan - Click configure
Bridge ports - Remove lan5 > Save
Click Interfaces tab
Scroll down and clikc “add new interface”
Protocol: Static address
Click create interface
Click “Edit” for lan5 now
IP4v address: 192.168.9.1 (you can use anything you want really)
IP4v netmask: 255.255.255.0
Firewall settings > drop down menu and select guest
DHCP Server > General Setup
Make sure “ignore interface” is unticked
You can set the Start/Limit/Lease time or leave it default
Save & Apply
If it all worked properly, you can connect a laptop to lan5 and you should be able to access guest (no access to local network if you have isolation enabled for guest) but access the internet.