I’m not sure as I’ve always stuck to WD drives. You should be able to Google the model + “shuck” and see if anyone else has done it. I know certain models either solder the controller to the drive or add it internally so that there’s no standard SATA port.
The larger clients should work the same as the micro sized ones, but don’t expect to get much more use out of them than maybe being able to store a drive internally as they’re typically full of proprietary connectors and stripped down motherboards. I’d only bother with one if you can buy it cheaper than one of the micro sized options like the Optiplex 3050
If you go to canyouseeme.org and test your Plex port does it show as open?
Your new ISP might have CG-NAT meaning you don’t get your own IPv4 address and can’t be connected to directly.
Also stream a movie from your phone while connected to cellular data and then check that stream on the Plex admin screen to see what it says. IIRC the Plex Relay is fixed at 1mbps