ITST 1.17 - Can't host games / portforwarding

[almatic]

I would really need some help if possible. Since I implemented my new router I can't host games. It says forward port 4321 on your router/firewall. I've done all that multiple times and played around with every possible setting of my router but still can't get through to host. I've also disabled windows firewall.
I'm on windows 10 and my router is a Asus RT-AC66U and wired connection.
Every tip is highly appreciated.

[151kbar151]

Same problem there.