This is how you would normally go about giving folder permissions to an IIS app pool:

Try opening IIS Manager, selecting App Pools on the left, and selecting the App Pool your Acumatica instance is installed in. Note its name.

Locate the folder you wish to allow permission to, and open it’s Properties. Switch to the security tab. Click Edit, and then click Add. In the box that comes up, type ‘IIS AppPool\*appPoolName*’ (replacing *appPoolName* with the actual app pool name, and without ‘ ‘). Click Ok. For good measure, restart the app pool (this will take Acumatica down for a bit).

BUT

Since you are using a secondary device, I’m not sure the above will work. You can try it. It should at least work for a local folder.

Just for closure:

This worked to give permission to the local C:/user/... folder that was previously denied.

However with a folder that lives on a Synology NAS, the properties/permissions/locations in windows do not include the local machine, so IIS AppPools can’t be granted permission. Only entities from the greater domain or local to the NAS.

One way to resolve this would be to create an AD user that has local admin permission on the host server (or something similar) and set the IIS AppPool to use that user account. Then permission can be given to that user on the NAS folder.

This can cause more complications to arise, but it is possible.