If the in-house users are not having any problems, I would first verify if the "timeouts" the remote users are getting are truly Access timeouts. Try to replicate the error over your own VPN connection.

If they are using the app via Citrix over a VPN, then I would ask how 'close; is your app to the Citrix box that generates the pages to the users on the other end. That is to say that I suspect your timeout is not Access but, rather, lag time between the endpoints of the network conversation. In fact, you may even be generating a true Access timeout, but it is probably an aritifact of the network slowness.

You can increase the timeout allowance between Access and SQL Server, but the rest of the equation has nothing to do with your application. Under 'File', choose 'Connection', then 'Advanced'.

Good luck!

Phil