I had the Multiple NAT message in Roon.
Here how I resolve that issue.
My network topology goes like this:
all my home network devices are connected to a dedicated TPLink router, behind my ISP provided modem/router.
I have NOTHING plugged in the ISP’s router except my TPLink router.
That is the cause of the double NAT I think.
I created a port forwarding rule in my ISP’s router to route the TCP 55000 port to 192.168.2.12, which is the IP of my TPLink router assigned by the ISP’s router.
And then it worked!
I am still trying to figure out if I am creating a huge security hole by doing the above.