You keep leaking out new, important info. This is a key statement:
It just won't get a valid IP address (says it's a self-assigned IP). Same problem trying to connect via a Macbook Pro. And same problem again trying to connect via an iMac. The common factor appears to be Mac OS (instead of iOS).
. You've had us scratching our heads as if it's restricted to a specific PC, rather than behaving consistently towards
any PC to which you have access.
Let's assume that the Xfinity public hotspot does discriminate between device types (routers certainly can identify the types of devices that connect, and can be set to reject inappropriate connections). After all, it's in Xfinity's interest to do so - those hotspots exist to serve Xfinity's customers when they're away from their home network. It's not there to
replace a home or office network. If the hotspot's neighbors permanently latch onto it, how much bandwidth might be left for transient Xfinity customers? It likely would reject a range extender, just as it would a PC, regardless of what OS that PC runs, and I'd expect you'd have a tough time connecting a wifi printer. One can argue that the hotspot should allow use by PCs while they're in true mobile use, but how would they make that distinction?
Overall, I think you have to adapt to the notion that Xfinity doesn't want you to use their hotspot in the manner you wish, and they're smart enough that they're not going to leave any gaping loopholes.