Mitel AMC does not restrict any IP addresses, there is no access list to adjust. The issue with with your settings or ISP.
Log into the command prompt of the MSL base, and troubleshoot connectivity there. The address used is sync.mitel-amc.com
Remember this is a vMCD, so it must be able to connect to AMC. 99% of the time this is a DNS or firewall issue, but could vary depending on your configuration, but a couple ping and nslookup commands in the terminal should narrow down the issue quite quickly.