The Mtronics guys seem a reasonable bunch so I hope they'll send you a replacement 🤞
Re: fuse, my measurements on the Sea Scout with the same rating motor you have and 3S LiPo never showed more than about 8Amps current at full chat so I have fitted a red 10A car blade fuse.
Clearly visible in the pics I've posted. My ESC is also rated 30A. But I wouldn't fit more than a 15A fuse at most with that motor. Some might go to 20A but while protecting the ESC Ok that may risk damage to the motor or wiring before it blows.
Was glad to see that the RX had survived OK 😉
Good luck with Mtronics👍 Cheers, Doug 😎