Can't Get DMZ Or Port Forwarding To Work ON ASUS DSL-AC68U

Hi all. Tried setting up a DMZ on the ASUS DSL-AC68U. However, it isn't working here. If I disconnect the ASUS DSL-AC68U and use the Sky UK fibre router with it's DMZ connected to my ASUS RC87U then everything works as expected. I simply want to use the DMZ on the ASUS DSL-AC68U and pass all internet traffic through to the RC87U. Have tried doing this with the firewall on and off on the ASUS DSL-AC68U and this makes no difference. My reason for wanting to use the
ASUS DSL-AC68U is that I can...

Can't Get DMZ Or Port Forwarding To Work ON ASUS DSL-AC68U