VPN IPSEC (FG to FG) can't bring up after disconnect - Need to re-build again

hi there,

need help please.

I'm trying to configure IPSEC vpn from FG60D to FG60D (let say, HQ to Branch)

I've successfully created VPN IPSEC by wizard. but whenever the branch disconnect/internet connection down, I can't bring up again.

both of them basically use dynamic IP public. But when Internet connection down, I restart and trace, IP public still same.

so here my question:

1. why this happen? and how to solve it, so I don't need to delete old IPSEC configuration, and create new one.

2. How to prevent above situation

3. how to make VPN IPsec I've created, auto connect whenever it disconnect?

4. I've use fortiddns feature. Why I can't put this dynamic DNS in remote gateway when I set up IPSEC vpn

I'm sorry for a lot of questions. really need help here.

thanks in advance.