Dear friend!
You need to set MAC inloop on the NMS.
On the NMS, open the NE Explorer of NE1, select the Ethernet board to be looped back, and choose Configuration > Ethernet Interface Management > Ethernet Interface from the Function Tree to set MAC inloop. In the WDM products, if the board does not support MAC inloop, you can perform inloop on the client-side optical interface.

There's a simpler way:
1. Connect the PC to the central site, and connect the Ethernet port to the MN_ETH port at the branch site using a network cable.

2. Run the ping command on the PC to ping the IP address of the peer NE.
More: How to test the availability of Ethernet services.
Thank you!