How to tell when the remote DC connection got cut off?

We have a remote office being out of internet connection for almost a week due to ISP's fiber cable incident. Now I need to investigate when the outage exactly began. To do that, first of all, I searched the firewall log since we have site-to-site VPN connection between the two firewalls. But I found out the log size limitation is too small so the log details of interest have been overwritten and not available.
Secondly, we have a DC in that office and there is site replication set up in ADSS as I can see. With that, do you know if there is log available somewhere regarding the remote DC connection getting cut off?

Thanks for your help.