Ok, I found another post with a similar issue:
https://community.talend.com/t5/Deployment/resolved-New-TAC-execution-server-with-status-down/td-p/1...
For anyone else that has this issue, I simply had to add an entry for the host IP and hostname to the /etc/hosts file.
e.g.
Update the hosts file
sudo vi /etc/hosts
Add the line below in as the second line (under 127.0.0.1 and above ::1)
10.20.20.10 HOSTNAME.NETWORK
Example hosts file:
127.0.0.1 localhost localhost.localdomain
10.20.20.10 HOSTNAME.NETWORK <------ add in this line here
::1 localhost localhost.localdomain
Hope that helps anyone else in the same position!