tl;dr: If you've got a PC, you can just buy a USB3 stick for six pounds and put Ubuntu on it. It's a little more work, but isn't difficult by any means, and it also has the advantage of not having to send your computer through domains such as "l.fastloading.co". Making a live USB stick for a Mac is a bit more involved, but still not rocket brainery.
TBH, I prefer to troubleshoot the problem with the operating system I intend to use. I have found situations in the past where Linux is fine, because reasons, and Windows isn't, because other reasons, and vice versa.
Macs are tricky, because their sealed, essentially disposable nature means you can't readily swap parts out to prove them, but I'd still be inclined to use macOS to do the troubleshooting, simply because it's the one OS that is guaranteed to be able to see and understand all the hardware in the machine, especially given Apple's predilection for Apple-special parts.
ttll;ddrr - don't give these shysters any money. There are better ways of achieving the same outcome.