Won't charge even after battery and lightning connector replacement

I have a 2 year old iPhone SE which until recently has functioned flawlessly. A few days ago I plugged it in to a USB port on a London bus at around 80% charge and noticed that it wasn't charging, and didn't recongnise that it was plugged in.

I tried to charge it later from a normal mains socket and it still wouldn't charge, but the phone continued to function until the battery died at the end of the day. Since then, it refused to charge or turn on.I tried various things including using different lighting cables (original Apple cables and 3rd party), leaving it to charge overnight, and heating it up with a hairdryer (I had read some things about cold weather causing charging problems). Still nothing.I replaced the battery and lighting connector earlier today, and the phone did turn on when I installed the battery and functioned fine, but still refused to charge, so clearly the phone is "working" i.e. not dead but just refuses to charge.Any suggestions on what to do next?

I've read some things about the "Tristar IC chip" apparently being the cause of similar issues on iPhones, and I think it may have something to do with the USB on the bus I plugged it in to being faulty and devlivering too much voltage or current?

I'm reasonably skilled at basic repairs but I'm not good at soldering etc. so I don't think I would be able to repair the Tristar myself if it turned out to be the cause of the issue.

What do others think?

Thanks,

Elshad