Seems the method of using certificate-based connection won’t work with non-Sophos firewall.
I had to switch to shared-phrase in order to make it work.
Not sure if that problem is due to old firmware/device on the other side or is it compliance issue.
However, I had to go on the VPN settings on both firewalls and make sure all settings at both sides are exactly the same.
Finally, I decided to switch back to pre-shared key instead of the certificate-based authentication between the two appliances.
The moment I set the key on the initiator, the tunle immediatly came up.