iPhone 6s won't charge past 3%!

Hello guys!

Few weeks ago I ran into a problem with my iPhone 6s. After a long day of using it without any problems , at night , when the battery had 15%, without dropping it all day or smashing it or anything like that, the phone just spontaneously restarted. It just restarted. After the phone booted up, the battery percentage was 1%. I immediately plugged the phone into the original Apple charger and cable , after that the battery percentage went to 3% and stuck there. I let it charge over night , woke up in the morning with 3%. Tried different chargers , cables.. I tried all the stepts on every single apple support on every topic and nothing. I had to go to a mobile service to see what happened to the battery , why so sudden without any reasons. The service guy changed the battery in front of me ready to pay him, when suddenly he tells me it's not the problem with the battery... With a new new battery it had still the same exact problem. From 1% , plugged in , jumps to 3% and stuck there. So no any topic tips , not any apple tips , not even changing the battery fixes it. Anyone has any ideea what can I do?

Thanks.