I have install a balance 210 in drop-in mode with shared public ip address, everything is working fine. The problem is that the remote Peplink Balance One Core is not able to establish a PepVPN tunnel with the balance 210.
PepVPN tunnel state stuck at starting on both side.
I simulated similar setup (FW 8.0.2) and can confirm that PepVPN cannot be established from a remote node with internal IP to local node with external IP on Balance working as Drop-in + shared IP.
We are checking internally if that scenario can be improved.
Now a workaround is (if my assumption is correct with your setup) - external IP on your Balance One Core. Then the tunnel can be initiated from Balance 210.
To be sure, please confirm what is your IP configuration at site B.
I got this new public IP from my ISP 210.10.8.37/32, the old IP address is 210.10.10.2/30.
I have setup drop-in mode with the same gateway on both side but the Balance 210 (in /30, Balance 210 device don’t allow network address in /32 on drop-in interface) failed to see firewall connected as client and the firewall lost internet connection.
Also i failed to add a static route to the to my LAN trough the next hop 210.10.10.2 Error : Device not in the same subnet with the WAN interface
I know this can be done but would allow the Balance to work normally, and provide NAT and DHCP for Site A, just as you have at site B. I would put the firewall in transparent mode instead of having the Balance in drop-in mode.