point agents to update cache and message relay

Hi Sophos User1175,

Thanks for reaching out to the Sophos Community Forum. 

To point an offline device to a specific Update Cache server, you will need to install using the command line. If the installation is already completed, you can instead append the "--registeronly" switch as shown below.

- SophosSetup.exe --messagerelays=x.x.x.x --registeronly

You can find more information on this in the following article. 
- Installer command-line options for Windows

If these devices are not internet-facing, I'd suggest ensuring you have the "Message Relay" component deployed to your Update Cache as well, but it sounds like this is already the case.

whats the difference between "update cache and message relay" as i only see the option messagerelay and not updatecache?

When you initially deploy the Update Cache feature to a device with Sophos Server Protection deployed, it will only serve out the updating files to the neighbouring devices. The endpoints will still require a network connection for the management of policies and reporting. The main purpose of an Update Cache is to alleviate internet bandwidth constraints.

If the endpoints do not have an internet connection and cannot reach Sophos Central directly, you will need to deploy the Message Relay feature to the server. This allows the management and reporting traffic to go through the Update Cache/Message Relay to reach Sophos Central.

Connection information on how the endpoints can reach the UC/MR is contained in the SophosSetup.exe package. If you have recently deployed a new cache, I'd suggest downloading a new installer package to use with the endpoints as well. 

I suggest checking out Sophos Techvids for a great overview of Update Caches and Message Relays. 
- Sophos Central: Configure Update Caches and Message Relays