The UNC path should not be the issue if the target server name can be resolved from the computer on which the package gets deployed, but rather the access to this path. You need to ensure that this package gets deployed under scanning credentials which need to have access to this path. Alternatively the currently logged on user might be working, if you are sure that all users which receive this package have access to the installer path.
Generally we recommend providing a package share and configure this under
Deployment\Security options. You can always refer to the package share location with the place holder {PackageShare} in your deployment packages, and you can define separate credentials to access the package share.
BTW, you can use the installer step for this as well. The UNC path to the installer EXE will be the
Install File and the parameters will be entered in the
Parameters field.