I experience lots of frustration with RD in 2008 server at my home network before it died last year. Most of it was none of my workstations had Windows Pro on any and couldn't connect. I since have Pro on several machines via W10. But no server to login to.
That's neither here nor there, just rambling.
Back to topic; I had lots of RD problems going back to 2000 server days. I found an easy way to fix it.
You'll have to go get that new W10 machine at the remote site (fly or drive there and bring it back to your location where the domain server lives). Or have someone there ship it to you. Plug it in to your main network, a different subnet port on your main network would be ideal. Attempt to remote into the W10 desktop and get it working like that. If it works; the network settings are ok on the internal network, and so you can look for exclusions or firewall blocks in your main server location as the probable cause of it failing to work at your remote location (either router/firewall equipment at the main location or at the remote location). Once you do admin logins to both main core routers and go through all the settings you can probably isolate and fix the problem. Ship the W10 machine back out to the remote site and have someone turn it on and plug it into the network. Retest; if it lets you remote in, you've fixed it! Easy, right?
If the W10 machine fails to let you remote in across a different subnet in your server location, physically move the W10 machine to a port on the core router which should be the same subnet as your 2008 server is plugged into. If you are out of ports on that core router, hang an Ethernet switch on an available port or even a hub, and plug the W10 machine in to that. Retest. Can you remote into the W10 machine with it plugged into the core router where the server Ethernet connection also lives? If you cannot remote in with the W10 machine on the same subnet, it's likely to be a problem with that machine.
You may have to do a manual rebuild using W10 Clean Install and reinstall all your programs and data manually. You can also use a different W10 machine and plug that in to the core router. If that 2nd PC works, but the original one doesn't, yet another reason to rebuild it.
If you cannot get any W10 machines to work on your network domain while plugged into the core router where the 2008 server lives; you probably have a topological problem with your network and you'll need to create a network diagram to fix it;
if you don't already have one. A sketch is a good idea; but you should really have a program like Visio at your disposal to create that diagram and keep it updated with any and all future changes to your network. You'd be surprised at how many of my customers could have solved their own problems if they just did this. It's only taught in universities and trade tech schools however, so the average Tech is often unaware of it's power to solve problems.
Hope that provides some further insight.
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