For the RAK2270 if you try to install on the Arduino IDE in North America, the download of the file at
is too slow and the board manager times-out almost every time. I was lucky enough to get it working on 1 windows machine in my computer lab. And of course it loads fine on a Linux computer.
A very bad hack is to put the file on your own github gitpages. Which are webpages served from github. Then plug the webpage link (not the github link as that will not work) into your Arduino Preferences and it works fine.
After that installing the STM32 board is fine.
DO NOT use someone else’s board.json file on their github site. That would be very bad from a security point of view.
It is very easy to make your own. Just use your browser to go to the above link. Copy the entire file to a Github location. Turn on GitPages to make a webpage of your Github site and find the link to the file. Put that webpage link into your Arduino IDE preferences.