EDIT: sorry, replied to wrong person
Hi @jan3
here is my idea
- no, you don't have public IP, and you set port forward FIRST on ISP router, or you setup DMZ on ISP router, whatever ...
- since your OWRT was only device attached on ISP router, there was "almost" static 192.168.1.x IP assigned for your old OWRT device
- so, your DMZ or FIRST port forward from ISP router worked as expected, because you set for ex: ISP 8080 -> 192.168.1.100 (OWRT) 8080 -> 192.168.2.100 (LAN) 8080
- you changed your device, and your "almost" static addres is changed, it is not 192.168.1.100 anymore, it is maybe 101 now
- your FIRST portfowrad from ISP router, or DMZ, point still to old 192.168.1.100 (old OWRT router)
- check your ISP router how it is set up
maybe this will help ?