Power bank charges phone slow

I have a Cygnett "ChargeUp Boost 2" 20k mAh power bank that is about 2 years old. I notice that it's charging fine but when I connect my phone, the phone says "Check your charger connection" which means the USB-A ports, or the connector, is bad. This is a pricey power bank so should I try and repair it? if so how do I take it apart at least?