Peplink Balance IPsec behind firewall

Hi all,

Im new to peplink here.Currently doing a simple setup in office to establish a ipsec vpn between 2 peplink.i able to establish ipsec vpn when both are in same network (siteA : /24& siteB : /24 ).

When i put siteB( ip adress change to /24 ) behind a firewall ( ) the ipsec vpn goes down. i already port foward UDP : 500,4500, TCP : 32015 but still down. any idea why ?