That's the issue with a lot of people they buy a bike and often can't find any where to have them repaired if not dealer supplied/supported bike which is often a mid drive type.
The guy over the road has had a step thru he bought for £400 used, it has been sitting in his shed for a while because it doesn't work. He wasted money and bought a new controller only (crap wave type) and still it wouldn't work, then he found out from his landlord ( a mate of mine) that I was an ebike guru .
He now has a EAPC stepthru that works perfectly , the guy is one happy guy .
I even pumped his tyres up , adjusted the crappy cable disc brakes and greased the cartidge BB for him.
I took the bike out before I gave it back to him for a test ride and surprisingly the bike rides very nicely but I did feel a little out of place riding a ladies style bike, it does help the bikes rides so nicely because I fitted it out with a KT system.
Bit of a pain teasing the wiring internal thru the frame .
With a non working or troublesome bike the biggest and best upgrade for a hub type bike is to simply rip everythng off and replace all with a KT upgrade for about £70 - £100, the best option to stop users fiddling and messing stuff up is to go basic with the LCD and use a LCD1 or LCD2. One can simply use a LCD3 first to set all the C settings for the controller then us ethe LCD1 or 2 as the final user option then they can't mess with much.