Hi, could someone please advice how we can setup two routers for Unifi (primary router for normal internet access, secondary router exclusive for VPN access)?

I tried setting up my routers. The primary router (Asus AC1300UHP) has access as expected, however the second router (Asus RT-N12HP) is not able to connect to internet (even before installing VPN on the router).

My setup is as below;
Modem ---> Wan port Primary Router ---> Lan port 1 (not Lan4 for IPTV) to WAN port Secondary Router.

What am I doing wrong? Thanks.

*Note: Connecting the secondary router to Modem directly does give internet access so I assume that the problem is connecting secondary router to primary router for internet access.