How to remotely access to TNAS mounted under secondary router?

When TNAS is mounted under the secondary router or multi-level router, the subnet of each level router is different, which makes the address of TNAS cannot be recognized and users can not conduct remote access. The followings are setting ways of secondary routers.
Step 1: Set up the primary router. Find DMZ project in the settings menu of primary router. Please enable DMZ Besides, input the IP address of the secondary router in the bar and click Save.
Step 2: Set up the secondary router. Find and initiate DMZ in the settings menu of the secondary router. Please enable DMZ and input the IP address of TNAS in the bar and click Save.