The SEPM (Symantec Endpoint Protection Manager) loses all communication when Dynamic Host Configuration Protocol (DHCP) changes the IP address.
If the IP address of the managing computer changes, clients do not receive the new address and continue (unsuccessfully) to attempt to connect to the old IP address.
The SEPM requires a static IP address. Dynamic IP address configuration on the SEPM is not supported or recommended.
Establish a static IP address for the SEPM and restore communications with clients manually using the Sylink.xml file:
How to change a Symantec Endpoint Protection client from unmanaged to managed
By default the clients will also try to resolve the name of the SEPM server if it can't connect via IP. As long as DNS records are updated and point to the new IP address then the client will continue communicating and should automatically receive the new communication file containing the new IP address.