Red flash plus loses power

I have 2 issues with it:

• It flips between the Apple logo then a red screen for 2 seconds then black then back to the logo
• When I plug it in it has power, but when I unplug it, it goes dead (so the battery is not getting charged)

The tech guy said the battery and bit that connects the charger are fine. Also when I do the home/power button thing, then home volume up, I get the restore screen. And also, it doesn’t show up nor charge when I plug it into my laptop.

So I’m thinking it has something to do with how the battery gets charged.

Any help would be much appreciated. It is a 5s but I don’t want to buy a new phone if I can help it.

Thanks.