I got the iPhone 6s Plus uk used like 4 days ago or so......phone is okay generally with a good battery too. But the problem is, when I charge it with powerbank, it won’t even increase. When I connect it to my laptop too maybe when copying some thing sha, it won't even increase no matter how long it's connected. I have used 2 powerbanks to test and I use the same cord that I use for charging. Note that it charges normal with a charger.

Pls what could possibly be wrong and what can I do?

Your phone does not have an issue, issue is you are using a substandard power bank. Get a good quality power bank and get your phone charging. I had same experience until I got a new age power bank that does the work.

I got this one for 6k

The one I tried tonight is Oraimo power bank oo. I will still again tomorrow though

HumanCalc:
The one I tried tonight is Oraimo power bank oo. I will still again tomorrow though

I suggest you try using another charging cable