I recently just worked through this problem with my RadRover. Rad support was great about it. They first sent a controller and when that didn't fic the problem they sent a new main wiring harness and that did fix the issue and the bike turns on just fine now. If I were you I'd pop the controller off the bike that isn't working and connect it to the working bike. If the bike powers up you now know that both the display and the controller are good so you've narrowed it down to the wiring. They covered mine under warranty but if you are out of warranty the harness was just $30.
That sounds like a lot of down time with support sending parts multiple times.
I have a 2 week old Rad City 4 <80 miles that lost all power while riding. Support doesn't seem to be in any hurry to resolve this early life failure. Before contacting support, I swapped batteries and thats not the issue, I also went through the whole connector check in the Help site troubleshooting. Still won't turn on.
Seems like a common issue, are there any wiring schematics available to check continuity and/or power at the harness connections? I'd like to get the bike running again, its getting close to a week since first contacting support and I had to follow-up with a call.. They did mention sending a replacement harness and controller that
"may" solve the issue but I don't have confirmation that anything is enroute. Swapping out parts blindly seems like rocks and clubs troubleshooting.