I just bought this power bank today and it's capacity is 5200 mah (FIL) and it's so light that's been giving me concerns that whether it's really up to that 5200mah.. please advise needed,, I bought it 2,500

A 10,000 mah polymer Fil power bank cost #5,000,so yours at the price you quoted is fair.
I noticed that Fil is the best indigenous power bank in Nigeria and their ratings is okay.
It takes longer time to charge than the Li-on type battery and it slowly depletes.
I am still using my Fil power bank that I bought last year and it still serves me well. Mine is also flat.

I recently bought a Samsung power bank rated at 30,000 mah that supports super fast charging but it hardly charges my 3,000 mah tablet twice.
Today I had to open the power bank. I noticed that 2 of the batteries were not connected to the panel which I did myself, only 3 were connected. The battery rating is 18650.

Below are reliable power banks in Nigeria

Fil
Romoss
Lumsing
Anker
New Age power bank
Surge

Below is a picture of the power bank and it is heavy.
Sometimes heavy power bank don't live up to their expectations. They can be stocked with heavy Metals, bag of sand or dead batteries. I was lucky to have 2 good new batteries in the Samsung pb I bought which I connected to the panel myself. I have fully charged my tablet and the 4 bars light indicator is intact.

thanks man