If I were you, I'd take one of them to the "man who unlocks phones, usually sited in the tawdry run-down part of the local town centre somewhere near the Cash Converters place" type affair and get it unlocked. That will be a cheap quick route to trying out networks.
I honestly have no idea how much a second hand dongle costs. A guess would be about £15.
If you want to improve the reception of the B593 you can plug in an external antenna which could be the cheaper "just outside the window" style thing, or the more ambitious directional roof antenna (what we have).
That said, while that improves the speed here, it's still perfectly reasonable without it (20 down, 20 up) as the B593 is a decent piece of kit.
However that's for later on if it is a workable solution. Or if you find no workable solutions and want to take a punt on spending quite a bit of cash.
Mobile broadband antennae and satellite dishes are not related.
If you have a Sky service that will carry on working separately. Though IIRC Sky require you to have a phone line to have the TV service so the box can spy on what you watch, and send data to Sky, and I don't know if that bit can work over 4G - it used to "dial out" like a dial up modem during the night. Likewise ordering films might or might not work over the 4G connection. I haven't had Sky for 15 years, so I'm probably out of date on that.
If the Sky box simply connects to your modem/router via Ethernet, then in theory, it should work with the B593, for example. You'd just plug the Ethernet cable into that instead like any other device.