puzzled at plainTransport.connect

plainTransport.connect(ip, port),how the mediasoup server get and connect ip if the ip is behind NAT such as 192.168.110.216.

Is that plain transport for sending RTP from client to mediasoup? from mediasoup to the client? or both?

from mediasoup to client.

Then you need to open a port in your NAT router and redirect. No magic is possible. This is how legacy RTP works.