Guess you’re a lucky guy!
I ran into the same issue these days and think I resolved it yesterday…
Have look there.
Regarding DMZ: You could (ab)use the GuestNet - but AFAIK only if you connect by WiFi to the Guest WiFi - I see no (easy) way to dedicate one of the LAN ports to the guest net.
In case you want an ethernet connection (faster, more stable, less latency) you’ll need to place the Pi in the LAN. In that case it’s IMO a bad idea to use the “DMZ” settings from your Flink, an unnecessary risk. Rather forward the single ports you need.
In case all your IT is behind the Flink you might consider setting up the Flink as DMZ host in your ISP-router so you only have to handle port forwarding at the flink instead of both.
HTH!