The error message is giving you the answer here, access is not authorised.

As tThe Computer is outside of the domain and it more than likely does not have access to SSRS. If the other domain is part of the same forest, you'll need to allow the domain SSRS is on to be able to trust the other domain, and then add the users(s) or group(s) of the other domain permissions to do whatever they need on SSRS.