This is a simplified illustration of your setup.
Internet --- 177.aaa.bbb.ccc <Askey Router> 192.168.15.1 --- 192.168.15.? <Ubiquiti Edgerouter> 192.168.1.1 --- 192.168.1.? <Roon Core>
Your ISP modem isn’t in bridge mode because it has a private IP address on the internal network. Therefore, it will have assigned an IP address to the Edgerouter: 192.168.15.? You need to identify the last number. This is usually listed in the router “connected devices” page.
Once you have this, you can forward port 55000 or 55002 (whatever you have chosen) to the Edgerouter.