Edit: I should have mentioned that the T-Mobile 5G Gateway ISP solution that your friend in Florida is using is their home internet provider. This is a home internet solution that is purely wireless. No fiber or cable pulled to your property. It is nice for homes that are not pre-wired for internet service, but there are limitations. Although their mobile devices may also be on a T-Mobile plan, they would work just fine with ARC if their home internet service provided a public IP address (like AT&T, Comcast, and Cox in most areas).
I use Roon ARC with Andriod Auto anytime we're in the car. Compatibility has less to do with the wireless carrier than it does with the internet provider one has at home. ARC requires that your ISP assign what's called a public IP address to the router you have at home. Because the world is running out of public IP addresses, they are becoming more expensive. For this reason, some internet providers have stopped providing them. Instead they assign a private address to customers and then translate connections through a smaller set of public addresses that they own. This reduces costs, but it also means that services like Roon's ARC will not work.
To solve this, Roon would have to set up what is called a proxy service in their cloud environment. The Roon Server that belongs to each subscriber would have to maintain a persistent connection to this proxy service. Mobile devices running ARC would connect to the same proxy, which would map the subscriber to the Roon Server that belongs to them.
IoT devices, like popular wireless security cameras and video doorbells, use a similar model. However, this would increase Roon's operating costs vs the current model where your mobile device connects directly to the router you have at home. Roon Labs would have to pay for operating this proxy service and for the bandwidth used by all subscribers using ARC. They may decide to support such a model, but if they do, watch for an increase in your subscription fee...or an additional fee for lifetime subscribers.
If your home internet provider does not provide a public IP address, unfortunately, your only practical option is to find a different provider. Highly technical folks may be able to get around this issue with VPN technology. I've not tried this with Roon, but I've heard that some folks have had luck getting VPNs to work. Even if that's possible, switching ISPs will free you up to spend more time listening to music and less time troubleshooting VPN issues!
Hope this makes some sense. Good luck!