You have to add the firewall rules to the app and the specific ports in which the app connects.
For app rules:
Go to Avast -> Protection -> Firewall -> Application options -> Create a New group (or search for the unified remote one if already created) and look for RemoteServerWin.exe in the Unified Remote folder, then allow connections for that .exe.
For port rules:
Go to Avast -> Protection -> Firewall -> Options -> Package rules -> Add a TCP/UDP new rule with Local port and Remote port at 9512 (Generally the app uses this port, you can also look the port up at the Server manager in the computer at the network section)
Some pages with pictures of the procedure:
Avast App Rules
Avast Port Rules
Hope it helps someone.