iPhone 6s stuck dead, on red charge icon

For starters, I have the same problem as this thread.

But here's my situation.

The few days, my iPhone 6s has only been charging to 95% or so overnight. Today (26th) it didn't want to charge all day, I would plug it in and immediately a message "This accessory may not be supported." Now the phone has died, and is stuck on the empty red battery logo.

I am using the cord and block that came with the phone.

I tried cleaning out the charging port, not much came out and nothing changed.

I tried booting into recovery mode and restoring with the latest iOS in iTunes, which it did, but right after restoring, it went back to the same dead battery sign.

I have tried multiple different charging methods, including, different cords, different blocks, USB 2.0/3.0, portable battery, everything I could think of.

My iPhone isn't too cold or too hot, its about 22ºC in my house.

Is this a charging port issue or a battery issue?

Your help is appreciated.