Bad SmartLink connection from some locations on some computers?

I've set up a WireGuard VPN for my club at a remote site in Raymond, WI, which has a solid 500b/500b fiber Internet connection. It seems to be working properly and when I connect, I see a 6400 and an 8400 installed at that site over the local (VPN) LAN. Both of those radios work fine.

However, when I try to connect to my Flex 6600 radio in North Carolina via SmartLink from inside that VPN, I encounter an issue:

1. The SmartLink connection screen shows only one red bar for Internet quality.
2. This happens consistently across multiple devices, including my Windows 11 laptop on-site and my Windows 10 machine at home.
3. Audio from North Carolina is periodically briefly interrupted.
4. Meanwhile, some users (including one sitting right next to me) connect perfectly with five green bars on his laptop.
5. Everything seems to work on computers not owned by me!

Some additional details:

• I have a Flex 8600 at a different site in Burlington, WI, to which I can connect flawlessly via SmartLink from all locations and devices.
• If I bypass SmartLink and connect directly to the NC 6600 via my personal VPN, it works without any issues.

To summarize, none of my computers will connect properly to the NC 6600 radio via SmartLink yet they will all connect to my BURLINGTON 8600 via Smartlink from the same locations. Most others don't seem to have this problem. NC has 1Gb/1Gb fiber.

This is a screenshot of what I see when attempting to connect NC 6600 via SmartLink through the Raymond VPN. I see the same thing when physically on site there.

Any thoughts on why this is happening? A diagram of the network we're trying to install is also below. The RPi's are not part of the radio communications, just control.