USB drive on a router doesn't permit Mac/Windows file sharing

Has anyone successfully gotten a router-mounted USB NAS device working on a PC/Mac network, with full read/write access to it from both machines?

I bought an ARRIS cable modem/wireless router ("ARRIS") that was certified "compatible" by Comcast/Xfinity, and one reason I chose it was because it has two USB ports, that theoretically should allow me to use a high capacity thumb drive for cross-platform (Windows+Mac), secure Network Attached Storage (NAS).

In practice, the ARRIS-mounted USB NAS only works great from the Windows PC:

Windows makes the USB NAS appear as a shared network folder, and, using Windows Explorer or any Windows application, I can create folders, read, write and drag/drop files and folders directly to the USB NAS.

But from the Mac Finder it is a different story:

I can read any files on the USB NAS that were previously placed there by the Windows PC, but...

I can't drag/drop or copy/paste any Mac files to the USB NAS.

I can't create folders on the USB NAS.

Strangely, the total disk space occupied by files/folders on the USB NAS that were previously placed there from Windows, is depicted as the total space available, when viewed from the Mac. Therefore, if I use the Windows computer to put i.e. 16MB of files on the 256GB USB NAS, and then try to add more files to the USB NAS from the MAC, I get, "The file XYZ can't be copied because there isn't enough disk space". The Mac thinks the USB NAS only has 16MB of disk space and is full, when in fact it is a nearly-empty 256 GB USB drive.

However, strangely, if I launch i.e. Word on the Mac, FROM WITHIN WORD ONLY, I can....

Freely create and save word.docs to the USB NAS

Freely create folders on the USB NAS from within the Word File/Save dialog, and save files into them.

These "Mac problems" occur both with my older MAC running OSX 10.6.8, and also from my new Mac running the latest High Sierra version of MAC OSX.

I wonder if the MAC problem would disappear if I could tell the MAC that the USB NAS is a shared network folder and get its permissions (from the MAC) to be the same as its permissions (from the Windows PC).

But I can't seem to make that happen.

From the Windows PC, the USB NAS is depicted as "Network\ARRIS LGW\[shared network folder name]" and the NAS's File Properties\Group or User Names shows 'Everyone' has 'Special permissions' to access that folder...but when I dig deeper, I can't seem to find out what those "Special Permissions" are.

In Windows, I tried changing 'Everyone’s' permissions to Full Control, but got “Access Denied” (even while logged in as Admin).

From the Mac, GET INFO \ Sharing and Permissions says the USB NAS's "Kind" is "Sharepoint", and the Mac has "custom access" to it, (but with no ability to change anything, and no way to find out what 'custom access' consists of.)

Although Arris tech support tried to help, they soon ran out of ideas (one tech simply hung up on me), suggesting it must be a Mac OS problem, and advised me to contact Apple Support. I think it's more likely a permissions problem generated by Arris's MAC-ignorant, windows-centric implementation of USB NAS, further compounded by both operating systems' bewildering mazes of Security/Permissions.

The end result is, although one of the big reasons I bought this Arris router was to have a router with a USB port so I could achieve 256GB of secure network attached storage with read-write access from both computers on my network, the Arris router has only delivered broken promises.

WIndows 7 Pro

Mac Mini, OSX 10.6.8

Mac Mini, OSX High Sierra 10.13.6