If you have issues connecting an AVG client to an AVG admin server ensure the following things are considered:
1. The server and the client must be running the same version. You cant run 2012 clients on 2016 server and visa versa.
2. Ensure you have specified port 4158 and in the admin console settings page try the FQDN is the server name alone does not work.
3. Try resetting the connection settings to defualt using the defualt button then re-enter the details.