Welcome to the forum
New - SP4 - off warranty? How did that happen?
Are you using the port on the Surface or the one on the power brick?
Some devices seem to be drawing more power than the USB port on the Surface can output. I would try a flash drive to see if you have the same issue. You may need a powered HUB though it would seem a phone should not pull that much power. Try a USB flash drive to see if you have the same problem
A slim possibility, but try a different usb cable (i know it worked previously, but you never know) My phone is constantly disconnecting due to usb cables. Only one i have actually creates a stable connection.
I just thought I'd clarify what I meant by slow charge. I dont mean slow in that it's not a fast charging protocol like on newer phones, but slow charge in that it is below even a normal charge. On my phone I get a warning message saying that it is slow charging with a big triangle and exclamation mark. If I were to watch YouTube while in this slow charge state, the battery would discharge.